接上篇：

利用类型，做函数模板的“重载”-CSDN博客

比较两种模板的写法

为什么左边不可行：

注意，左边的写法的第二个模板参数，是默认参数的形式。为何这里采取了默认参数的形式呢，本意是想让编译器来走sfine的流程，用户不用指明具体的类型。但就是这一点让最后的编译失败

当编译到test451(1);时，实例化成了

??$test451@HH@@YAXH@Z，也就是void __cdecl test451<int,int>(int)

template <typename Sequence, typename=int>
    void test451(Sequence x)
{
    std::cout << "1" << std::endl;
}

当编译到test451(1.0);时，按理说应该生成：void __cdecl test451<double,double>(double)

template <typename Sequence, typename=double>
    void test451(Sequence x)
{
    std::cout << "1" << std::endl;
}

但可能编译器不这么想，却给了一个“error C2995: “void test451(Sequence)”: 函数模板已经定义”的编译错误。明明void __cdecl test451<int,int>(int)和void __cdecl test451<double,double>(double)并不一样啊。

用C++ Insights分析

祭出C++ Insights吧，让它来分析分析下面的代码，看能否给出更多信息：

打开下面的链接

 C++ Insights

#include <cstdio>
#include <iostream>
#include <type_traits>template <typename Sequence, typename=typename std::enable_if<std::is_same<Sequence, int>::value, int >::type>void test451(Sequence x)
{std::cout << "1" << std::endl;
}template <typename Sequence, typename =typename std::enable_if<std::is_same<Sequence, double>::value, int >::type >void test451(Sequence x)
{std::cout << "1" << std::endl;
}template <typename Sequence, typename =typename std::enable_if<std::is_same<Sequence, std::wstring>::value, int >::type >void test451(Sequence x)
{std::cout << "1" << std::endl;
}int main()
{test451(1);test451(1.0);std::wstring str;test451(str);
}

给出的结论如下：

/home/insights/insights.cpp:13:5: error: template parameter redefines default argument
   13 |     typename std::enable_if<std::is_same<Sequence, double>::value >::type>
      |     ^

意思是，在实例化double的情况的时候，提示默认参数被重定义了。

/home/insights/insights.cpp:6:5: note: previous default template argument defined here
    6 |     typename std::enable_if<std::is_same<Sequence, int>::value >::type>
      |     ^

之前实例化的在这里。（int版本）

/home/insights/insights.cpp:14:10: error: redefinition of 'test451'
   14 |     void test451(Sequence x)
      |          ^
/home/insights/insights.cpp:7:10: note: previous definition is here
    7 |     void test451(Sequence x)
      |          ^

double版本的test451重新定义了int版本的test451。（所以报错）

果然是默认参数的问题。用int实例化的时候，默认参数是int，

/home/insights/insights.cpp:13:5: error: template parameter redefines default argument13 |     typename std::enable_if<std::is_same<Sequence, double>::value, int >::type >|     ^
/home/insights/insights.cpp:6:5: note: previous default template argument defined here6 |     typename std::enable_if<std::is_same<Sequence, int>::value, int >::type>|     ^
/home/insights/insights.cpp:14:10: error: redefinition of 'test451'14 |     void test451(Sequence x)|          ^
/home/insights/insights.cpp:7:10: note: previous definition is here7 |     void test451(Sequence x)|          ^
/home/insights/insights.cpp:28:5: error: no matching function for call to 'test451'28 |     test451(1);|     ^~~~~~~
/home/insights/insights.cpp:14:10: note: candidate template ignored: requirement 'std::is_same<int, double>::value' was not satisfied [with Sequence = int]14 |     void test451(Sequence x)|          ^
/home/insights/insights.cpp:29:5: error: no matching function for call to 'test451'29 |     test451(1.0);|     ^~~~~~~
/home/insights/insights.cpp:14:10: note: candidate template ignored: substitution failure [with Sequence = double, $1 = typename std::enable_if<std::is_same<double, double>::value, int>::type]14 |     void test451(Sequence x)|          ^
/home/insights/insights.cpp:32:5: error: no matching function for call to 'test451'32 |     test451(str);|     ^~~~~~~
/home/insights/insights.cpp:14:10: note: candidate template ignored: requirement 'std::is_same<std::basic_string<wchar_t, std::char_traits<wchar_t>, std::allocator<wchar_t>>, double>::value' was not satisfied [with Sequence = std::wstring]14 |     void test451(Sequence x)|          ^
5 errors generated.
Error while processing /home/insights/insights.cpp.

其他链接：

c++模板元编程std::enable_if详解-CSDN博客