为什么在 C++ 概念示例中添加对 std::array 的支持会导致“调用时没有匹配的函数”编译错误？

Question

我正在使用cppreference.com - Constraints and Concepts (Since C++20)中的示例学习 C++ 概念，我将在此处将其重现为：

template <typename T> 
requires std::integral<T> || std::floating_point<T>
constexpr double average(std::vector<T> const &vec) {
    const double sum = std::accumulate(vec.begin(), vec.end(), 0.0);        
    return sum / vec.size();
}

int main() 
{
    std::vector sVec { 1, 2, 3, 4, 5};
    assert(average(sVec) == 3); 
}

我添加了 std::array 的情况：

template <typename T, int size>  // add size
requires std::integral<T> || std::floating_point<T>
constexpr double average(const std::array<T, size> &arr) {  // use std::array<T, size> &arr) instead
    const double sum = std::accumulate(arr.begin(), arr.end(), 0.0);        
    return sum / arr.size();
}

int main() 
{
    std::array sArr { 1, 2, 3, 4, 5};
    assert(average(sArr) == 3);
}

现在编译出现错误：

C:\Users\Cheng\OneDrive\Documents\Proj\HackerRank\Cpp\concept_introduction.cpp:65:19: error: no matching function for call to 'average(std::array<int, 5>&)'
   65 |     assert(average(sArr) == 3);
      |            ~~~~~~~^~~~~~
C:\Users\Cheng\OneDrive\Documents\Proj\HackerRank\Cpp\concept_introduction.cpp:14:18: note: candidate: 'template<class T>  requires (integral<T>) || (floating_point<T>) constexpr double average(const std::vector<T>&)'
   14 | constexpr double average(const std::vector<T> &vec) {
      |                  ^~~~~~~
C:\Users\Cheng\OneDrive\Documents\Proj\HackerRank\Cpp\concept_introduction.cpp:14:18: note:   template argument deduction/substitution failed:
C:\Users\Cheng\OneDrive\Documents\Proj\HackerRank\Cpp\concept_introduction.cpp:65:19: note:   'std::array<int, 5>' is not derived from 'const std::vector<T>'
   65 |     assert(average(sArr) == 3);
      |            ~~~~~~~^~~~~~
C:\Users\Cheng\OneDrive\Documents\Proj\HackerRank\Cpp\concept_introduction.cpp:22:18: note: candidate: 'template<class T, int size>  requires (integral<T>) || (floating_point<T>) constexpr double average(const std::array<T, size>&)'
   22 | constexpr double average(const std::array<T, size> &arr) {
      |                  ^~~~~~~
C:\Users\Cheng\OneDrive\Documents\Proj\HackerRank\Cpp\concept_introduction.cpp:22:18: note:   template argument deduction/substitution failed:
C:\Users\Cheng\OneDrive\Documents\Proj\HackerRank\Cpp\concept_introduction.cpp:65:19: note:   mismatched types 'int' and 'long long unsigned int'
   65 |     assert(average(sArr) == 3);
      |            ~~~~~~~^~~~~~

Build finished with error(s).

为什么？

Answer 1

的第二个模板参数std::array是 astd::size_t而不是int。

将第一行：更改template <typename T, int size>
为：template <typename T, std::size_t size>

应该解决这个问题。模板类型必须匹配。