为什么在约束中找到模板，而不是函数？ C＆＃x2B;＆＃x2B; 20

发布于 2025-02-04 22:10:28 字数 956 浏览 1 评论 0原文

我正在学习C ++ 20个概念和约束，但我不明白为什么在这种情况下被认为是未定义的，但它似乎在需要子句中工作正常。这里怎么了？为什么？

如果我删除了分号，则编译器说u不是模板

error: expected unqualified-id before ‘;’ token
   11 | requires std::integral<T> || std::floating_point<T>;
      |                                                    ^
main.cpp:12:28: error: ‘T’ was not declared in this scope
   12 | constexpr double average(U<T> const &it) {

#include <numeric>
#include <vector>
#include <iostream>
#include <concepts>

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

int main() {
    std::vector ints { 1, 2, 3, 4, 5};
    std::cout << average(ints) << '\n';
}

原文

I'm learning about C++20 concepts and constraints and I don't understand why in this case T is considered undefined, yet it seems to work fine in the requires clause. What's wrong here? and why?

If I remove the semicolon, the compiler says U is not a template

error: expected unqualified-id before ‘;’ token
   11 | requires std::integral<T> || std::floating_point<T>;
      |                                                    ^
main.cpp:12:28: error: ‘T’ was not declared in this scope
   12 | constexpr double average(U<T> const &it) {

#include <numeric>
#include <vector>
#include <iostream>
#include <concepts>

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

int main() {
    std::vector ints { 1, 2, 3, 4, 5};
    std::cout << average(ints) << '\n';
}

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打小就很酷 2025-02-11 22:10:28

请阅读有关 Container Concept 完整的解决方案。这是仅仅显示方向的部分解决方案。

#include <numeric>
#include <vector>
#include <iostream>
#include <concepts>

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

int main() {
    std::vector ints { 1, 2, 3, 4, 5};
    std::cout << average(ints) << '\n';
}

please read about container concept for the full solution. here is a partial solution just to show a direction.

#include <numeric>
#include <vector>
#include <iostream>
#include <concepts>

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

int main() {
    std::vector ints { 1, 2, 3, 4, 5};
    std::cout << average(ints) << '\n';
}

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