C++:为什么'operator + ='定义但不是'operator +'用于字符串？

Question

为什么operator+=定义std::string但operator+未定义？请参阅下面的我的MWE(http://ideone.com/OWQsJk).

#include <iostream>
#include <string>
using namespace std;

int main() {  
    string first;
    first = "Day";
    first += "number";
    cout << "\nfirst = " << first << endl;

    string second;
    //second = "abc" + "def";       // This won't compile
    cout << "\nsecond = " << second << endl;
    return 0;
}

Answer 1

您需要将其中一个原始字符串文字转换为std::string显式.你可以像其他已经提到过的那样做:

second = std::string("abc") + "def";

或者使用C++ 14,您将能够使用

using namespace std::literals;
second = "abc"s + "def";
// note       ^

Answer 2

那些不是std::strings,它们是const char *.试试这个:

 second = std::string("abc") + "def";