在模板中输入条件

2022-01-20 00:00:00 conditional templates c++

在给定 C++ 模板类型的情况下，是否可以只构建部分代码?这将是一个湖泊:

Is it possible to build only some part of the code given the type of the template in C++ ? It would be something lake that :

#include <iostream>

using namespace std;

template<typename T>
void printType(T param)
{
    #if T == char*
        cout << "char*" << endl;
    #elif T == int
        cout << "int" << endl;
    #else
        cout << "???" << endl;
    #endif
}

int main()
{
    printType("Hello world!");
    printType(1);
    return 0;
}

推荐答案

从 C++17 开始，有一种方法可以使用 if-constexpr 来做到这一点.以下编译自 clang-3.9.1、gcc-7.1.0 和最近的 MSVC 编译器 19.11.25506 也可以通过选项/std:c++17 处理.

Since C++17 there is a way to do exactly this with if-constexpr. The following compiles since clang-3.9.1, gcc-7.1.0, and recent MSVC compiler 19.11.25506 handles well too with an option /std:c++17.

#include <iostream>
#include <type_traits>

template<typename T>
void printType(T)
{
    if constexpr (std::is_same_v<T, const char*>)
        std::cout << "const char*" << std::endl;
    else if constexpr (std::is_same_v<T, int>)
        std::cout << "int" << std::endl;
    else
        std::cout << "???" << std::endl;
}

int main()
{
    printType("Hello world!");
    printType(1);
    printType(1.1);
    return 0;
}

输出:

const char*
int
???

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