C ++打印模板容器错误（错误：“ operator<<”含糊不清的重载）理解？

问题描述

我想编写模板函数，该函数可以打印 std :: vector ，std :: list之类的容器。

I want to write template function which can print container like std::vector, std::list.

下面是我的函数，只是重载<< 。

Below is my function, just overload <<.

template<typename Container>
std::ostream& operator<<(std::ostream& out, const Container& c){
    for(auto item:c){
        out<<item;
    }
    return out;
}

测试代码如下：

int main(){
    std::vector<int> iVec{5, 9, 1, 4, 6};
    std::cout<<iVec<<std::endl;
    return 0;
}

输出：

59146

我想在每个空格中添加一个空格值（输出类似于 5 9 1 4 6 ），因此我将函数更改为：

And I want to add a space string in each value(output like 5 9 1 4 6), so I change the function to:

template<typename Container>
std::ostream& operator<<(std::ostream& out, const Container& c){
    for(auto item:c){
        out<<item<<" ";
    }
    return out;
}

然后出现错误：

merror: ambiguous overload for 'operator<<' (operand types are 'std::basic_ostream<char>' and 'const char [2]')
         out<<item<<" ";

我知道<< 可以输出

int a = 0;
double f = 0.3;
std::string s = "123";
std::cout<<a<<f<<s<<std::endl;

那么为什么会出现上述错误？并且有什么办法解决它？

So Why get the above error ? And is there any way to solve it?

我已经看到了这个问题中'operator<<'的重载，但我仍然不清楚。

I have see this question Ambiguous overload for ‘operator<<’ in ‘std::cout << but I still can't understand clearly.

所有代码：

#include <iostream>
#include <vector>

template<typename Container>
std::ostream& operator<<(std::ostream& out, const Container& c){
    for(auto item:c){
        out<<item;
        // out<<item<<" "; // error
    }
    return out;
}

int main(){
    std::vector<int> iVec{5, 9, 1, 4, 6};
    std::cout<<iVec<<std::endl;
    return 0;
}

推荐答案

声明 template< typename容器> 可能很危险，因为此模板包含所有变量类型 int ， char 等。由于此编译器不知道要使用哪个 operator<< 。

Declaring template<typename Container> could be dangerous as this template includes 'all' variable types int, char etc. Due to this compiler does not know which operator<< to use.

为了仅采用容器类型变量，请使用模板模板。
这是为您工作的代码

In order to take only container type variables use template of templates.Here is working code for you

template<typename T, template <typename, typename> class Container>
std::ostream& operator<<(std::ostream& out, const Container<T, std::allocator<T>>& c) {
    for (auto item : c) {
        out << item << " ";
    } 
    return out;
}

int main()
{
    cout << "Hello world" << endl;
    int arr[] = { 0,3,6,7 };
    vector<int> v(arr, arr+4);
    cout << v << endl;
    return 0;
}

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