C++11 += 运算符在 RHS 有多个字符串或字符时不起作用

Question

I am using c++11 . When I use the += operator for concating strings or characters, it does not work, however = , works.

eg I used all the below test cases independently, ie separately.

    string s="abdddddd";
    string ss="";
    ss+=s[0];//working
    ss+=s[0]+s[1]; //not working     output: Ã
    ss+="hi"+s[2];  //not working      no output
    ss+='d'+'c';  //not working      output: З
    ss+="hi"+"string"; //not working  error: invalid operands of types ‘const char [3]’ and ‘const char [7]’ to binary ‘operator+’

    string another="this";
    ss+=another+'b'; //working
    ss+="hi"+another;//working
    ss+=("hi"+s[3]); //not working
    ss=ss+"hi"+s[3]; //working
    ss=ss+"hi"+"this"; //working

Also adding brackets doesn't work. So, I want to know why it doesn't work with strings, it works with adding integers.

Answer 1

Your problem is not += , it is + .

For example in the line ss+=s[0]+s[1]; , s[0] and s[1] are of type char . Adding them will add their integer representations (ASCII codes) and concatenate the character this sum represents to ss .

You see the same happing in ss+='d'+'c'; , where you are giving char literals explicitly.

Only use + to concatenate std::string with std::string or one std::string with a character or string literal. You cannot concatenate two string literals or two characters or a string literal with a character. (In all of these cases + has different semantics than string concatenation or isn't even valid syntax.)