The other day I found out that MSVC's std::map move constructor doesn't have noexcept:
map(map&& _Right) : _Mybase(_STD move(_Right)) {}
This means that it can throw when moved and it's not eligible for move_if_noexcept which is used in vector reallocation's. I filed a report but it got rejected. Take care when you want map's to be moveable (which can be a factor compared to copying all nodes).
Aggregate initialization brought us many good things. There is though one nasty pitfall with std::vector. std::vector can be initialized with a size_t to specify its initial size. There is a potential ambiguity since the vector also accepts size_t as its element type:
#include <cassert>
#include <vector>
int main()
{
std::vector<size_t> vec0(5); // vector size is set
std::vector<size_t> vec1{5}; // vector size isn't set directly but vector is initialized with 1 element
assert(vec0.size() == 5);
assert(vec1.size() == 1);
std::vector<size_t> vec2{vec0.size()}; // even worse: vectors do not have the same size now
return 0;
} This is especially a problem with 32 bits code where size_t is an int typedef. A solution is that the C++ committee resolves the ambiguity e.g. by introducing a 2 argument overload for vector constructor (with an extra enum). Until then it's watching out
Boost libraries are a great addition to C++. However anyone working with Boost lirbaries know that they explode your compile times. For example include adjacency_list for Boost.Graph lead to the following spam:
graph\adjacency_list.hpp
unordered_set.hpp
unordered/unordered_set.hpp
core/explicit_operator_bool.hpp
functional/hash.hpp
container_hash/hash.hpp
move/move.hpp
move/detail/config_begin.hpp
move/utility.hpp
move/detail/config_begin.hpp
move/detail/config_end.hpp
move/algorithm.hpp
move/detail/config_begin.hpp
move/detail/config_end.hpp
move/detail/config_end.hpp
unordered/detail/set.hpp
unordered/detail/implementation.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
core/allocator_traits.hpp
core/allocator_access.hpp
core/pointer_traits.hpp
core/bit.hpp
cstdint.hpp
swap.hpp
type_traits/is_base_of.hpp
type_traits/is_nothrow_move_assignable.hpp
type_traits/has_trivial_move_assign.hpp
type_traits/enable_if.hpp
type_traits/is_nothrow_move_constructible.hpp
type_traits/is_nothrow_swappable.hpp
type_traits/make_void.hpp
unordered/detail/fwd.hpp
predef.h
predef/language.h
predef/language/stdc.h
predef/version_number.h
predef/make.h
predef/detail/test.h
predef/language/stdcpp.h
predef/make.h
predef/language/objc.h
predef/make.h
predef/language/cuda.h
predef/make.h
predef/architecture.h
predef/architecture/alpha.h
predef/make.h
predef/architecture/arm.h
predef/make.h
predef/architecture/blackfin.h
predef/make.h
predef/architecture/convex.h
predef/make.h
predef/architecture/e2k.h
predef/make.h
predef/architecture/ia64.h
predef/make.h
predef/architecture/loongarch.h
predef/make.h
predef/architecture/m68k.h
predef/make.h
predef/architecture/mips.h
predef/make.h
predef/architecture/parisc.h
predef/make.h
predef/architecture/ppc.h
predef/make.h
predef/architecture/ptx.h
predef/make.h
predef/architecture/pyramid.h
predef/make.h
predef/architecture/riscv.h
predef/make.h
predef/architecture/rs6k.h
predef/make.h
predef/architecture/sparc.h
predef/make.h
predef/architecture/superh.h
predef/make.h
predef/architecture/sys370.h
predef/make.h
predef/architecture/sys390.h
predef/make.h
predef/architecture/x86.h
predef/architecture/x86/32.h
predef/make.h
predef/architecture/x86.h
predef/architecture/x86/32.h
predef/architecture/x86/64.h
predef/make.h
predef/architecture/x86.h
predef/architecture/x86/32.h
predef/architecture/x86/64.h
predef/architecture/x86/64.h
predef/architecture/z.h
predef/make.h
predef/compiler.h
predef/compiler/borland.h
predef/make.h
predef/compiler/clang.h
predef/make.h
predef/compiler/comeau.h
predef/make.h
predef/compiler/compaq.h
predef/make.h
predef/compiler/diab.h
predef/make.h
predef/compiler/digitalmars.h
predef/make.h
predef/compiler/dignus.h
predef/make.h
predef/compiler/edg.h
predef/make.h
predef/compiler/ekopath.h
predef/make.h
predef/compiler/gcc_xml.h
predef/make.h
predef/compiler/gcc.h
predef/compiler/clang.h
predef/make.h
predef/compiler/greenhills.h
predef/make.h
predef/compiler/hp_acc.h
predef/make.h
predef/compiler/iar.h
predef/make.h
predef/compiler/ibm.h
predef/make.h
predef/compiler/intel.h
predef/make.h
predef/compiler/kai.h
predef/make.h
predef/compiler/llvm.h
predef/compiler/clang.h
predef/make.h
predef/compiler/metaware.h
predef/make.h
predef/compiler/metrowerks.h
predef/make.h
predef/compiler/microtec.h
predef/make.h
predef/compiler/mpw.h
predef/make.h
predef/compiler/nvcc.h
predef/make.h
predef/compiler/palm.h
predef/make.h
predef/compiler/pgi.h
predef/make.h
predef/compiler/sgi_mipspro.h
predef/make.h
predef/compiler/sunpro.h
predef/make.h
predef/compiler/tendra.h
predef/make.h
predef/compiler/visualc.h
predef/compiler/clang.h
predef/make.h
predef/detail/comp_detected.h
predef/compiler/watcom.h
predef/make.h
predef/library.h
predef/library/c.h
predef/library/c/_prefix.h
predef/detail/_cassert.h
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\cassert
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
predef/library/c/cloudabi.h
predef/make.h
predef/library/c/gnu.h
predef/make.h
predef/library/c/uc.h
predef/make.h
predef/library/c/vms.h
predef/make.h
predef/library/c/zos.h
predef/make.h
predef/library/std.h
predef/library/std/_prefix.h
predef/detail/_exception.h
predef/library/std/cxx.h
predef/make.h
predef/library/std/dinkumware.h
predef/make.h
predef/library/std/libcomo.h
predef/make.h
predef/library/std/modena.h
predef/make.h
predef/library/std/msl.h
predef/make.h
predef/library/std/roguewave.h
predef/make.h
predef/library/std/sgi.h
predef/make.h
predef/library/std/stdcpp3.h
predef/make.h
predef/library/std/stlport.h
predef/make.h
predef/library/std/vacpp.h
predef/make.h
predef/os.h
predef/os/aix.h
predef/make.h
predef/os/amigaos.h
predef/make.h
predef/os/beos.h
predef/make.h
predef/os/bsd.h
predef/os/macos.h
predef/os/ios.h
predef/make.h
predef/make.h
predef/make.h
predef/os/bsd/bsdi.h
predef/os/bsd.h
predef/os/bsd/bsdi.h
predef/os/bsd/dragonfly.h
predef/os/bsd.h
predef/os/bsd/bsdi.h
predef/os/bsd/dragonfly.h
predef/os/bsd/free.h
predef/os/bsd.h
predef/os/bsd/bsdi.h
predef/os/bsd/dragonfly.h
predef/os/bsd/free.h
predef/os/bsd/open.h
predef/os/bsd.h
predef/os/bsd/bsdi.h
predef/os/bsd/dragonfly.h
predef/os/bsd/free.h
predef/os/bsd/open.h
predef/os/bsd/net.h
predef/os/bsd.h
predef/os/bsd/bsdi.h
predef/os/bsd/dragonfly.h
predef/os/bsd/free.h
predef/os/bsd/open.h
predef/os/bsd/net.h
predef/os/bsd/net.h
predef/os/bsd/open.h
predef/os/bsd/net.h
predef/os/bsd/free.h
predef/os/bsd/open.h
predef/os/bsd/net.h
predef/os/bsd/dragonfly.h
predef/os/bsd/free.h
predef/os/bsd/open.h
predef/os/bsd/net.h
predef/os/bsd/bsdi.h
predef/os/bsd/dragonfly.h
predef/os/bsd/free.h
predef/os/bsd/open.h
predef/os/bsd/net.h
predef/os/cygwin.h
predef/make.h
predef/os/haiku.h
predef/make.h
predef/os/hpux.h
predef/make.h
predef/os/irix.h
predef/make.h
predef/os/ios.h
predef/os/linux.h
predef/make.h
predef/os/macos.h
predef/os/os400.h
predef/make.h
predef/os/qnxnto.h
predef/make.h
predef/os/solaris.h
predef/make.h
predef/os/unix.h
predef/make.h
predef/os/vms.h
predef/make.h
predef/os/windows.h
predef/make.h
predef/detail/os_detected.h
predef/other.h
predef/other/endian.h
predef/make.h
predef/library/c/gnu.h
predef/os/macos.h
predef/os/bsd.h
predef/os/bsd/bsdi.h
predef/os/bsd/dragonfly.h
predef/os/bsd/free.h
predef/os/bsd/open.h
predef/os/bsd/net.h
predef/platform/android.h
predef/make.h
predef/architecture.h
predef/other/wordsize.h
predef/architecture.h
predef/make.h
predef/other/workaround.h
predef/platform.h
predef/platform/android.h
predef/platform/cloudabi.h
predef/make.h
predef/platform/mingw.h
predef/make.h
predef/platform/mingw32.h
predef/make.h
predef/platform/mingw64.h
predef/make.h
predef/platform/windows_uwp.h
predef/make.h
predef/os/windows.h
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\shared\ntverp.h
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\shared\ntverp.ver
predef/detail/platform_detected.h
predef/platform/windows_desktop.h
predef/make.h
predef/os/windows.h
predef/platform/windows_uwp.h
predef/platform/windows_phone.h
predef/make.h
predef/os/windows.h
predef/platform/windows_uwp.h
predef/platform/windows_server.h
predef/make.h
predef/os/windows.h
predef/platform/windows_uwp.h
predef/platform/windows_store.h
predef/make.h
predef/os/windows.h
predef/platform/windows_uwp.h
predef/platform/windows_system.h
predef/make.h
predef/os/windows.h
predef/platform/windows_uwp.h
predef/platform/windows_runtime.h
predef/make.h
predef/os/windows.h
predef/platform/windows_phone.h
predef/platform/windows_store.h
predef/platform/ios.h
predef/os/ios.h
predef/hardware.h
predef/hardware/simd.h
predef/hardware/simd/x86.h
predef/hardware/simd/x86/versions.h
predef/hardware/simd/x86_amd.h
predef/hardware/simd/x86_amd/versions.h
predef/hardware/simd/arm.h
predef/hardware/simd/arm/versions.h
predef/hardware/simd/ppc.h
predef/hardware/simd/ppc/versions.h
predef/version.h
utility/addressof.hpp
unordered/unordered_set_fwd.hpp
functional/hash_fwd.hpp
scoped_ptr.hpp
smart_ptr/scoped_ptr.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
checked_delete.hpp
smart_ptr/detail/sp_disable_deprecated.hpp
smart_ptr/detail/operator_bool.hpp
graph/graph_traits.hpp
pending/property.hpp
type_traits.hpp
type_traits/common_type.hpp
type_traits/decay.hpp
type_traits/remove_bounds.hpp
type_traits/remove_extent.hpp
type_traits/detail/mp_defer.hpp
type_traits/conjunction.hpp
type_traits/copy_cv.hpp
type_traits/copy_cv_ref.hpp
type_traits/copy_reference.hpp
type_traits/disjunction.hpp
type_traits/extent.hpp
type_traits/floating_point_promotion.hpp
type_traits/has_bit_and.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_bit_and_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_bit_or.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_bit_or_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_bit_xor.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_bit_xor_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_complement.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_dereference.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_divides.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_divides_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_equal_to.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_greater.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_greater_equal.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_left_shift.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_left_shift_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_less.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_less_equal.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_logical_and.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_logical_not.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_logical_or.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_minus.hpp
type_traits/has_minus_assign.hpp
type_traits/has_modulus.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_modulus_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_multiplies.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_multiplies_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_negate.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_new_operator.hpp
type_traits/has_not_equal_to.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_nothrow_destructor.hpp
type_traits/has_trivial_destructor.hpp
type_traits/has_plus.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_plus_assign.hpp
type_traits/has_post_decrement.hpp
type_traits/detail/has_postfix_operator.hpp
type_traits/has_post_increment.hpp
type_traits/detail/has_postfix_operator.hpp
type_traits/has_pre_decrement.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_pre_increment.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_right_shift.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_right_shift_assign.hpp
type_traits/detail/has_binary_operator.hpp
type_traits/has_trivial_move_constructor.hpp
type_traits/has_unary_minus.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_unary_plus.hpp
type_traits/detail/has_prefix_operator.hpp
type_traits/has_virtual_destructor.hpp
type_traits/is_abstract.hpp
type_traits/is_complex.hpp
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\complex
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\ymath.h
type_traits/is_compound.hpp
type_traits/is_copy_assignable.hpp
type_traits/is_noncopyable.hpp
type_traits/is_float.hpp
type_traits/is_list_constructible.hpp
type_traits/is_member_object_pointer.hpp
type_traits/is_object.hpp
type_traits/is_scoped_enum.hpp
type_traits/negation.hpp
type_traits/is_signed.hpp
type_traits/is_stateless.hpp
type_traits/is_trivially_copyable.hpp
type_traits/is_union.hpp
type_traits/is_unscoped_enum.hpp
type_traits/is_unsigned.hpp
type_traits/is_virtual_base_of.hpp
type_traits/make_signed.hpp
type_traits/make_unsigned.hpp
type_traits/rank.hpp
type_traits/remove_all_extents.hpp
type_traits/remove_cv_ref.hpp
type_traits/type_identity.hpp
type_traits/integral_promotion.hpp
type_traits/promote.hpp
pending/detail/property.hpp
type_traits/same_traits.hpp
graph/graph_mutability_traits.hpp
graph/graph_selectors.hpp
property_map/property_map.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
concept_archetype.hpp
property_map/vector_property_map.hpp
property_map/property_map.hpp
smart_ptr/shared_ptr.hpp
smart_ptr/detail/shared_count.hpp
smart_ptr/bad_weak_ptr.hpp
smart_ptr/detail/sp_counted_base.hpp
smart_ptr/detail/sp_has_gcc_intrinsics.hpp
smart_ptr/detail/sp_has_sync_intrinsics.hpp
smart_ptr/detail/sp_counted_base_std_atomic.hpp
smart_ptr/detail/sp_typeinfo_.hpp
smart_ptr/detail/sp_counted_impl.hpp
cstdint.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
smart_ptr/detail/spinlock_pool.hpp
smart_ptr/detail/spinlock.hpp
smart_ptr/detail/spinlock_std_atomic.hpp
smart_ptr/detail/yield_k.hpp
smart_ptr/detail/sp_thread_pause.hpp
smart_ptr/detail/sp_thread_sleep.hpp
smart_ptr/detail/sp_win32_sleep.hpp
smart_ptr/detail/operator_bool.hpp
smart_ptr/detail/local_sp_deleter.hpp
smart_ptr/detail/local_counted_base.hpp
graph/detail/edge.hpp
functional/hash.hpp
container_hash/hash.hpp
graph/properties.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
graph/property_maps/constant_property_map.hpp
graph/property_maps/null_property_map.hpp
graph/named_graph.hpp
functional/hash.hpp
container_hash/hash.hpp
optional.hpp
optional/optional.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
optional/bad_optional_access.hpp
none.hpp
none_t.hpp
utility/compare_pointees.hpp
utility/result_of.hpp
utility/detail/result_of_variadic.hpp
optional/optional_fwd.hpp
optional/detail/optional_config.hpp
optional/detail/optional_factory_support.hpp
optional/detail/optional_aligned_storage.hpp
optional/detail/optional_trivially_copyable_base.hpp
optional/detail/optional_reference_spec.hpp
optional/detail/optional_relops.hpp
optional/detail/optional_swap.hpp
pending/container_traits.hpp
next_prior.hpp
iterator/advance.hpp
unordered_map.hpp
unordered/unordered_map.hpp
functional/hash.hpp
container_hash/hash.hpp
unordered/detail/map.hpp
unordered/unordered_map_fwd.hpp
functional/hash_fwd.hpp
C:\Program Files (x86)\Microsoft Visual Studio\2019\Enterprise\VC\Tools\MSVC\14.29.30133\include\unordered_set
graph/detail/adjacency_list.hpp
range/irange.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
range/iterator_range.hpp
range/iterator_range_core.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
range/functions.hpp
range/begin.hpp
range/config.hpp
range/iterator.hpp
range/range_fwd.hpp
range/mutable_iterator.hpp
range/detail/extract_optional_type.hpp
range/detail/msvc_has_iterator_workaround.hpp
range/const_iterator.hpp
range/end.hpp
range/detail/implementation_help.hpp
range/detail/common.hpp
range/detail/sfinae.hpp
range/size.hpp
range/size_type.hpp
range/difference_type.hpp
range/has_range_iterator.hpp
range/concepts.hpp
iterator/iterator_concepts.hpp
concept/detail/concept_def.hpp
concept/detail/concept_undef.hpp
range/value_type.hpp
range/detail/misc_concept.hpp
range/detail/has_member_size.hpp
cstdint.hpp
assert.hpp
C:\Program Files (x86)\Windows Kits\10\Include\10.0.19041.0\ucrt\assert.h
cstdint.hpp
range/distance.hpp
iterator/distance.hpp
range/empty.hpp
range/rbegin.hpp
range/reverse_iterator.hpp
range/rend.hpp
range/algorithm/equal.hpp
range/detail/safe_bool.hpp
range/iterator_range_io.hpp
graph/graph_concepts.hpp
graph/numeric_values.hpp
graph/buffer_concepts.hpp
typeof/typeof.hpp
typeof/message.hpp
typeof/decltype.hpp
concept/detail/concept_def.hpp
concept/detail/concept_undef.hpp
concept/detail/concept_def.hpp
concept/detail/concept_undef.hpp
graph/detail/adj_list_edge_iterator.hpp
graph/adjacency_iterator.hpp
assert.hpp
...about 600+ files
The other day I stumbled upon code which accidentally used a double as end condition:
#include <cstdint>
#include <cstdlib>
constexpr double g_dW = 10;
int main()
{
int res = 0;
for (int n = 0; n < g_dW; ++n)
{
res += std::rand(); // use rand just to have a valid loop
}
return res;
}
One would think that the optimizer Visual Studio 2019 16.13 is smart enough to use integer comparison but this is not the case. VS2019 issues a relative expensive integer to double conversion and comparison:
for (int n = 0; n < g_dW; ++n)
00007FF7C9A01011 movsd xmm6,mmword ptr [__real@4024000000000000 (07FF7C9A02240h)]
00007FF7C9A01019 mov edi,ebx
00007FF7C9A0101B nop dword ptr [rax+rax]
{
res += std::rand();
00007FF7C9A01020 call qword ptr [__imp_rand (07FF7C9A02188h)]
00007FF7C9A01026 inc edi
00007FF7C9A01028 add ebx,eax
00007FF7C9A0102A movd xmm0,edi
00007FF7C9A0102E cvtdq2pd xmm0,xmm0
00007FF7C9A01032 comisd xmm6,xmm0
00007FF7C9A01036 ja main+20h (07FF7C9A01020h)
}
return res;
The solution is simple to use an integer as block condition:
constexpr int g_dW = 10;
Anyone who does serious application development would notice that standard C++ lacks extensive library support on the level of e.g. Java or .NET. For example many applications need the following:
There are 3th party libraries who fulfill this gap but it's always more difficult to get that up and running than if it would be stock present. It would therefore hard to recommend to use C++ for new application development.
Boost fulfill some of these gaps. Their quality of libraries vary; some of them are even abandoned and not maintained anymore. Still I would like the following libraries to be incorporated in std as well:
Windows spotlight is responsible for those nice aesthetic pictures on the logon screen. Somehow it was broken on my machine and I was deprived of this feature. Even when one selected the option (under 'settings', 'personalization' 'lock screen'), it would jump to normal picture mode.
Google didn't help at first (their search results become worse and worse over the years) until I got the tip from 'Software Test Tips': some necessary background apps where disabled. After enabling the app 'settings' (under 'settings'; 'privacy'; 'background apps') rebooting and waiting some time it works now.
In some heavily invoked code the following construct was used:
interface ITable
{
virtual std::string GetData() const = 0;
};
class Table : public ITable
{
public:
virtual std::string GetData() const override
{
return std::string{"1"};
}
};
class TableCt : public Table
{
public:
long GetDataCt() const
{
const std::string strData = GetData(); // watch out
return std::stol(strData);
}
};
Client code invokes 'GetDataCt', e.g.:
void f()
{
TableCt tbl;
tbl.GetDataCt();
}
in the real case TableCt wasn't the final class but GetData will not be overriden anywhere else. The code is functional correct but it takes a performance hit since base class function 'GetData' is invoked through vtable:
TableCt tbl;
tbl.GetDataCt();
00007FF653F710BC lea rdx,[rsp+30h]
00007FF653F710C1 lea rcx,[tbl]
00007FF653F710C6 call rax
On closer thought this is logical since the compiler must deal with 'GetData' being overwritten by a derived class. Therefore explicitly calling the base class function is needed to tell the compiler that it must use the base class:
class TableCt : public Table
{
long GetDataCt() const
{
return std::stol(__super::GetData());
}
};
Using the keyword final could also do the trick too although it depends then on the cleverness of the compiler.
The other day I was profiling a SSE optimized distance function and the more optimized form was 3 times as slow as the basic variant. The code was a bit like this (skipping the SSE variant):
template <typename T>
class Point
{
public:
constexpr Point (T x, T y);
constexpr T GetX () const;
constexpr T GetY () const;
private:
T m_x;
T m_y;
};
template <typename T>
constexpr Point<T>::Point(T x, T y)
: m_x(x)
, m_y(y)
{
}
template <typename T>
constexpr T Point<T>::GetX() const
{
return m_x;
}
template <typename T>
constexpr T Point<T>::GetY() const
{
return m_y;
}
// explicit (DLL) instantiation
template class __declspec(dllexport) Point<double>;
double DistSqr(const Point<double>& rpt1, const Point<double>& rpt2)
{
const double dx = rpt1.GetX() - rpt2.GetX();
const double dy = rpt1.GetY() - rpt2.GetY();
return (dx * dx) + (dy * dy);
}
It turned out that exported functions take a major performance hit; much larger than the two multiplications in 'DistSqr' function. The explicit exported template instantiation exports all functions; even constexpr and inline functions.The reason that calling exported function is slower:
Removing the export the function was 3 times faster than without the exported attribute. The accessor functions 'GetX' are then inlined.
Lessons learned:
Note: accessor functions like 'GetX' are prescribed by OOP but be aware of their potential performance cost.
There is sometimes the need to translate one enumerate value to another. For example let's have the case of enum 'A' and 'B':
enum A
{
eA0 = 0,
eA1,
eA2,
};
enum B
{
eB0 = 0,
eB1,
eB2,
};
A simple translation without asserting on non existing values could work as follow:
B Translate(A eA)
{
B e = eB0;
switch (eA)
{
case eA0: e = eB0; break;
case eA1: e = eB1; break;
case eA2: e = eB2; break;
}
return e;
}
This is a straightforward one to one mapping. With Compiler explorer one can see that the Visual Studio 2019 compiler translates this in a bunch of compare and jump statements:
B Translate(A) PROC test ecx, ecx je SHORT $LN4@Translatesub ecx, 1 je SHORT $LN5@Translatecmp ecx, 1 jne SHORT $LN4@Translatemov eax, 2 ret 0 $LN5@Translate: mov eax, 1 ret 0 $LN4@Translate: xor eax, eax ret 0 BTranslate(A) ENDP
In this case the value are exact the same so a more optimal solution would be that it just uses the value:
B Translate2(A eA)
{
return static_cast<B>(eA);
}
In Compiler explorer it can be seen now that this is denser and faster:
BTranslate2(EA) PROC mov eax, ecx ret 0 EB(EA) ENDPTranslate2
It seems that Visual Studio doesn't recognize this optimization by itself.
Above cast can be dangerous if values do not correspond. One can trap for this and for accidental changes by adding some static_asserts
enum A
{
eAbegin = 0,
eA0 = ABegin,
eA1,
eA2,
eAend
};
B Translate2(A eA)
{
static_assert(eAbegin == eBbegin);
static_assert(eAend == eBend);
// or (verbose and requires updating after adding an enum):
static_assert(eA0 == eB0);
static_assert(eA1 == eB1);
static_assert(eA2 == eB2);
return static_cast<B>(eA);
} auto was once introduced to circumvent complex type deductions inside template functions. Lately however many C++ guru's think that one should use it as much as possible since the compiler always deduces the correct type.
Counter arguments are as follow:
The canonical example of incorrect deduction is use in a for loop:
std::vector<std::string> vec;
for (auto a : vec)
{
}
Here the variable 'a' will deduced to having type std::string so that a copy of the string is made on every loop. This is sub optimal and it would be better if the type was const std::string&. This is a known idiom and people are advised to use 'const auto&' but it illustrates that blindly using auto doesn't imply 'automatic' or 'best type' always.
Another example is in the use of doubles vs floats. This can be important since there is a tradeoff between precision (double) and performance (float for GPU's).
This is a personal preference. Use of auto may reduce readability. Consider the following example:
auto a = GetPerson();
From these statement alone one cannot see if the return person is a (smart) pointer, a value or something else. This makes a difference; for example in case of using it in multi threaded environments.
Consider the alternative where the type is made explicit:
std::shared_ptr<Person> ptr = GetPerson();
Its now clear that it was a pointer type and that the memory of returned object is taken care of.
Consider another case:
Person* p = GetPerson();
As a reader of the code it's clear now that one needs to question the ownership. Raw pointers are not recommended but sometimes they are unavoidable (e.g. external library like TensorRT).
The readability can somewhat mitigated by use of Hungarian notation but in case of auto it's even more dangerous that the prefix will not reflect the (deduced) type.
auto a = GetSerialize();
It turned out that this the returned object was some kind of interface (again from TensorRT) which needed to be destroyed. This was completely unnoticed until it was written out:
IHostMemory* p = GetSerialize();
auto is a very good addition to the C++ language. It has its merit in original use case and with complex types where the actual type is not important. Iterators are a canonical example. As usual with any technique it can also hurt if you overuse it.