c++ - 如何编写插入运算符函数模板？

Question

我正在尝试为插入运算符编写一个函数模板，而不是一堆类似的重载。冗余的重载版本有效，但是当我尝试将它们组合在一个函数模板中时，编译器提示有歧义。例如:

#include <iostream>
#include <list>

class fooBar
{
public:
    fooBar(int iVal): iValue(iVal) {}
    int getValue() {return iValue;}
    
private:
    int iValue;

};

class foo
{
public:
    foo()
    {
        for(int i = 0; i < 64; i++)
            lstFooBars.push_back(fooBar(i));
    }
    std::list<fooBar>& getList()
    {
        return lstFooBars;
    }

private:
    std::list<fooBar> lstFooBars;
    
};

class bar
{
public:
    bar()
    {
        for(int i = 0; i < 64; i++)
            lstFooBars.push_back(fooBar(i));
    }
    std::list<fooBar>& getList()
    {
        return lstFooBars;
    }

private:
    std::list<fooBar> lstFooBars;
    
};

std::ostream& operator<<(std::ostream& osOut, fooBar& fbrFooBar)
{
    osOut << fbrFooBar.getValue();

    return osOut;
}

template <typename T> std::ostream& operator<<(std::ostream& osOut, T& tContainer)
{
    for(fooBar fbrFooBar: tContainer.getList())
        osOut << "[" << fbrFooBar << "] ";

    return osOut;
}

int main()
{
    foo fooFoo;
    bar barBar;

    std::cout << std::endl << fooFoo << std::endl << std::endl;
    std::cout << std::endl << barBar << std::endl << std::endl;

    return 0;
}

...编译器告诉我:

test.cpp: In function ‘std::ostream& operator<<(std::ostream&, T&)’:
test.cpp:63:9: error: ambiguous overload for ‘operator<<’ (operand types are ‘std::ostream’ {aka ‘std::basic_ostream<char>’} and ‘const char [2]’)
   63 |   osOut << "[" << fbrFooBar << "] ";
      |   ~~~~~ ^~ ~~~
      |   |        |
      |   |        const char [2]
      |   std::ostream {aka std::basic_ostream<char>}

为什么当您针对每种情况一遍又一遍地重载相同的函数时它会起作用并且它不会像这样编译？我在这里缺少什么？

Answer 1

您无意中为 const char* 添加了一个可能的重载:

template<typename T>
std::ostream& operator<<(std::ostream& osOut, T& tContainer)

如果您使用 SFINAE 缩小范围，它应该可以工作。

此重载仅适用于具有 getList() 成员函数的类型，例如:

template<typename T, typename U = decltype(std::declval<T>().getList())>
std::ostream& operator<<(std::ostream& osOut, T& tContainer)