使用
原件
给定一个std::array<std::variant<>>，我得到总字节大小(例如{float,float,uint32_t}=4+4+4=12)，然后我想得到void*形式的数组(传递给Vulkan)。
我认为我不需要为此进行堆分配，尽管我一直纠结于如何将计算出的大小作为模板参数传递。
Repl：https://repl.it/@JonathanWoollet/Humongous独特定义
错误：
> clang++-7 -pthread -std=c++17 -o main main.cpp
main.cpp:42:30: error: constexpr variable 'size' must be initialized by a
constant expression
constexpr uint32_t const size = arrSize<num>(arr);
^ ~~~~~~~~~~~~~~~~~
main.cpp:17:34: note: non-constexpr function 'accumulate<const
std::variant<unsigned int, float> *, unsigned long, (lambda at
main.cpp:19:9)>' cannot be used in a constant expression
return static_cast<uint32_t>(std::accumulate(arr.cbegin(),arr.cend(),
^
main.cpp:42:37: note: in call to 'arrSize(arr)'
constexpr uint32_t const size = arrSize<num>(arr);
^
/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../../include/c++/8/bits/stl_numeric.h:146:5: note:
declared here
accumulate(_InputIterator __first, _InputIterator __last, _Tp __init,
^
main.cpp:44:21: error: no matching function for call to 'toVoidPtr'
void* voidArr = toVoidPtr<num,size>(arr);
^~~~~~~~~~~~~~~~~~~
main.cpp:25:7: note: candidate template ignored: invalid explicitly-specified
argument for template parameter 'Size'
void* toVoidPtr(std::array<std::variant<uint32_t,float>,Num> const& arr) {
^
2 errors generated.
compiler exit status 1
(使用C++20编译会导致相同的错误)
我看到编译时间相当于std:：accumulate()，但是https://en.cppreference.com/w/cpp/algorithm/accumulate清楚地显示了constexpr的定义：
template< class InputIt, class T, class BinaryOperation >
constexpr T accumulate( InputIt first, InputIt last, T init,
BinaryOperation op );
我通过获得尺寸
template <uint32_t Num>
constexpr uint32_t arrSize(std::array<std::variant<uint32_t,float>,Num> const& arr) {
auto var_size = [](auto const& var) -> size_t {
using T = std::decay_t<decltype(var)>;
return sizeof(T);
};
return static_cast<uint32_t>(std::accumulate(arr.cbegin(),arr.cend(),
std::size_t{ 0 },
[var_size](std::size_t acc, auto const var) { return acc + std::visit(var_size,var); }
));
}
我通过转换为void*
template <uint32_t Num, uint32_t Size>
void* toVoidPtr(std::array<std::variant<uint32_t,float>,Num> const& arr) {
size_t byteCounter = size_t{0};
std::array<std::byte,Size> bytes;
std::for_each(arr.cbegin(),arr.cend(), [&](auto const& var) {
std::visit([&] (auto const& var) {
using T = std::decay_t<decltype(var)>;
std::memcpy(bytes.data()+byteCounter,static_cast<void const*>(&var),sizeof(T));
byteCounter += sizeof(T);
},var);
});
return static_cast<void*>(bytes.data);
}
我想按照这样的订单：
int main() {
std::cout << "Hello World!n";
const uint32_t num = 3U;
std::array<std::variant<uint32_t,float>,num> arr = { 1.34F, 3.76F, 124U };
constexpr uint32_t const size = arrSize<num>(arr);
std::cout << "size: " << size << std::endl;
void* voidArr = toVoidPtr<num,size>(arr);
}
我不确定我在这里错过了什么，而且我似乎也弄不明白。如果有任何帮助，我将不胜感激。
(我错过了任何东西，请留下评论，我会尝试添加它)
使用gcc 10.2.0&c++20
按照Werner Henze的建议更新为使用兼容的构建工具，并更新我的本地项目以使用gcc 10.2.0Henze和c++20后，我得到了不同的错误。
[build] In file included from C:UsersjonatProjectsgpu_blasc++testExampleTests.cpp:2:
[build] c:usersjonatprojectsgpu_blasc++example.hpp: In instantiation of 'ComputeApp<NumPushConstants>::ComputeApp(const char*, uint32_t, const uint32_t*, float**, std::array<std::variant<unsigned int, float>, NumPushConstants>, std::array<unsigned int, 3>, std::array<unsigned int, 3>) [with unsigned int NumPushConstants = 1; uint32_t = unsigned int]':
[build] C:UsersjonatProjectsgpu_blasc++testExampleTests.cpp:41:5: required from here
[build] c:usersjonatprojectsgpu_blasc++example.hpp:367:95: in 'constexpr' expansion of 'Utility::pushConstantsSize<1>(pushConstant)'
[build] c:usersjonatprojectsgpu_blasc++example.hpp:102:53: in 'constexpr' expansion of 'std::accumulate<const std::variant<unsigned int, float>*, long long unsigned int, Utility::pushConstantsSize<1>::<lambda(std::size_t, auto:43)> >((& pushConstants)->std::array<std::variant<unsigned int, float>, 1>::cbegin(), (& pushConstants)->std::array<std::variant<unsigned int, float>, 1>::cend(), 0, <lambda closure object>Utility::pushConstantsSize<1>::<lambda(std::size_t, auto:43)>{<lambda closure object>Utility::pushConstantsSize<1>::<lambda(const auto:42&)>()})'
[build] c:usersjonatprojectsgpu_blasc++example.hpp:367:38: error: 'pushConstant' is not a constant expression
[build] 367 | constexpr uint32_t const sizeTester = Utility::pushConstantsSize<NumPushConstants>(pushConstant);
[build] | ^~~~~~~~~~
[build] c:usersjonatprojectsgpu_blasc++example.hpp: In instantiation of 'ComputeApp<NumPushConstants>::ComputeApp(const char*, uint32_t, const uint32_t*, float**, std::array<std::variant<unsigned int, float>, NumPushConstants>, std::array<unsigned int, 3>, std::array<unsigned int, 3>) [with unsigned int NumPushConstants = 3; uint32_t = unsigned int]':
[build] C:UsersjonatProjectsgpu_blasc++testExampleTests.cpp:849:5: required from here
[build] c:usersjonatprojectsgpu_blasc++example.hpp:367:95: in 'constexpr' expansion of 'Utility::pushConstantsSize<3>(pushConstant)'
[build] c:usersjonatprojectsgpu_blasc++example.hpp:102:53: in 'constexpr' expansion of 'std::accumulate<const std::variant<unsigned int, float>*, long long unsigned int, Utility::pushConstantsSize<3>::<lambda(std::size_t, auto:43)> >((& pushConstants)->std::array<std::variant<unsigned int, float>, 3>::cbegin(), (& pushConstants)->std::array<std::variant<unsigned int, float>, 3>::cend(), 0, <lambda closure object>Utility::pushConstantsSize<3>::<lambda(std::size_t, auto:43)>{<lambda closure object>Utility::pushConstantsSize<3>::<lambda(const auto:42&)>()})'
[build] c:usersjonatprojectsgpu_blasc++example.hpp:367:38: error: 'pushConstant' is not a constant expression
[build] mingw32-make[2]: *** [testCMakeFilesExampleTests.dirbuild.make:82: test/CMakeFiles/ExampleTests.dir/ExampleTests.cpp.obj] Error 1
[build] mingw32-make[1]: *** [CMakeFilesMakefile2:315: test/CMakeFiles/ExampleTests.dir/all] Error 2
[build] mingw32-make: *** [makefile:159: all] Error 2
[build] Build finished with exit code 2
我在这里打了更多的比赛，但我找不到问题，我再次非常感谢任何帮助。
由于我无法提供Repl来最低限度地再现这一点，因此这里有一个指向CMake项目的链接，该项目的当前分支会产生以下错误：https://github.com/JonathanWoollett-Light/gpu_blas/tree/Compiler-update/c%2B%2B
cppreference accumulate表示accumulate自C++20以来只是constexpr，而不是在C++17中。因此，您需要切换到C++20。
如果你想知道哪个编译器已经支持constexpr accumulate，你可以看看cppreference编译器的支持，并搜索"；用于数值算法的constexpr"/P1645R1。例如clang需要libc++12。