如何在cpp中使用std::enable_if从模板处理此类问题

Question

我正在尝试创建将任何元素或元素序列写入输出流的函数。我使用 std::enable_if 使用模板来完成。函数有两个版本：第一个用于处理序列，第二个用于处理单个元素，例如 char。代码如下：

#include <iostream>
#include <list>
#include <type_traits>
#include <array>

const int maximum_array_length = 10;
const int maximum_element_length = 50;

/*below are functions which must write any element or their sequences to an any output stream*/

/*template function for non arithmetic types like char and so on.*/
template <typename T, typename O, typename std::enable_if<std::negation<typename std::is_arithmetic<T>::value>::value, bool>::type = 0>
void cstoio(T container, O& out) {
    for (auto& i : container) {
        if (!i.empty() && *i.begin()) {
            for (auto& j : i)
                if (j)
                    out << j;

            out << '\n';
        }
    }
}

/*same function but for arithmetic types*/
template <typename T, typename O, typename std::enable_if<std::is_arithmetic<T>::value, bool>::type = 0>
void cstoio(T ae, O& out) {
    out << ae << '\n';
}

/*test function that launchs cstoio-function 3 times with different arguments*/
void test_f() {

    /*create arrays of chars*/
    std::array<char, maximum_element_length> ca1{ "aaa" };
    std::array<char, maximum_element_length> ca2{ "bbb" };
    std::array<char, maximum_element_length> ca3{ "ccc" };
    std::array<char, maximum_element_length> ca4{ "  " };
    std::array<char, maximum_element_length> ca5{ " ddd" };

    /*create an array of arrays of chars*/
    std::array<std::array<char, maximum_element_length>, maximum_array_length> a{ ca1, ca2, ca3, ca4, ca5 };

    /*create a list of strings*/
    std::list<std::string> l{ "abc","", " " ,"zzz", "end" };

    /*test launchs*/
    cstoio(a, std::cout); //launch with an array
    cstoio(l, std::cout); //launch with a list
    cstoio('a', std::cout); //launch with a single char

}

但是我遇到了多个编译时错误：

Severity    Code    Description Project File    Line    Suppression State
Error   C2664   'void cstoio(char,std::ostream)': cannot convert argument 1 from 'std::array<std::array<char,50>,10>' to 'char' HelloWorld  E:\Studying\Programming\Cpp\Repos\NiceWeather\HelloWorld\Header.h   43  

Severity    Code    Description Project File    Line    Suppression State
Error   C2664   'void cstoio(char,std::ostream)': cannot convert argument 1 from 'std::list<std::string,std::allocator<_Ty>>' to 'char'
        with
        [
            _Ty=std::string
        ]   HelloWorld  E:\Studying\Programming\Cpp\Repos\NiceWeather\HelloWorld\Header.h   44  


Severity    Code    Description Project File    Line    Suppression State
Error (active)  E0304   no instance of overloaded function "cstoio" matches the argument list   HelloWorld  E:\Studying\Programming\Cpp\Repos\NiceWeather\HelloWorld\Header.h   48  

Severity    Code    Description Project File    Line    Suppression State
Error (active)  E0304   no instance of overloaded function "cstoio" matches the argument list   HelloWorld  E:\Studying\Programming\Cpp\Repos\NiceWeather\HelloWorld\Header.h   49

Answer 1

您的 std::negation 调用是错误的。成功

std::enable_if<std::negation<std::is_arithmetic<T>>::value, bool>::type

或者只是简单地

std::enable_if<!std::is_arithmetic<T>::value, bool>::type

Demo

如何在cpp中使用std::enable_if从模板处理此类问题

How to deal with such problems from templates using std::enable_if in cpp

c++

templates

enable-if