C++ 模板 function 在 Visual Studio 2019 中不起作用 [错误 2668]

Question

在通用交换 function 中使用模板函数时出错：

#include <iostream>
#include <complex>

using namespace std;

template <class T>
inline void swap(T& d, T& s)
{
    T temp = d;
    d = s;
    s = temp;
}

int main()
{
    int m = 5, n = 10;
    double x = 5.3, y = 10.6;

    complex<double> r(2.4, 3.5), s(3.4, 6.7);

    cout << "inputs: " << m << "," << n << endl;
    swap(m, n);
    cout << "outputs: " << m << "," << n << endl;

    cout << "double inputs: " << x << "," << y << endl;
    swap(x, y);
    cout << "double outputs: " << x << "," << y << endl;

    cout << "complex inputs: " << r << "," << s << endl;
    swap(r, s);
    cout << "complex outputs: " << r << "," << s << endl;

    return 0;
}

有人可以在这里发现错误吗？ 我在三个调用函数处收到错误：

错误 C2668 'swap'：对重载 function 的模糊调用

---

我改成std::格式，即代码：

#include <iostream>
#include <complex>

template <class T>
inline void swap(T& d, T& s)
{
    T temp = d;
    d = s;
    s = temp;
}

int main()
{
    int m = 5, n = 10;
    double x = 5.3, y = 10.6;

    std::complex<double> r(2.4, 3.5), s(3.4, 6.7);

    std::cout << "inputs: " << m << "," << n << std::endl;
    swap(m, n);
    std::cout << "outputs: " << m << "," << n << std::endl;

    std::cout << "double inputs: " << x << "," << y << std::endl;
    swap(x, y);
    std::cout << "double outputs: " << x << "," << y << std::endl;

    std::cout << "complex inputs: " << r << "," << s << std::endl;
    swap(r, s);
    std::cout << "complex outputs: " << r << "," << s << std::endl;

    return 0;
}

现在我得到了完整的错误：

SwapFunction.cpp(55,14): error C2668: 'swap': ambiguous call to overloaded function
SwapFunction.cpp(32,13): message : could be 'void swap<std::complex<double>>(T &,T &)'
        with
        [
            T=std::complex<double>
        ]
1>C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\VC\Tools\MSVC\14.28.29333\include\utility(102,19): message : or       'void std::swap<std::complex<double>,0>(_Ty &,_Ty &) noexcept(<expr>)' [found using argument-dependent lookup]
        with
        [
            _Ty=std::complex<double>
        ]

所以我只在std::complex<double>调用 function 中遇到错误，接下来可以做什么？

Answer 1

using namespace std可能是这里的罪魁祸首，因为它将std::swap拉入全局命名空间。

因此，您可以删除它并完全限定相关标识符（推荐）或重命名您自己的swap function。