为什么“+=”运算符在添加多个字符串时不起作用？

Question

when I'm trying to concatenate a string variable with more than one string with this operator(+=) it rises an error.

NOTE: The other two strings should not be stored in a variable.

string str="hello";
    
str = str + " " + "world";
cout << str << endl;    //hello world
    
str += " " + "world";
cout << str;            //error              //why?
                        //*but in java it works*

Answer 1

Because you're trying to add two string literals. The expression:

" " + "world"

in C++ is of type

const char* + const char*

const char* has no overloaded operator+. But standard strings do overload operator+ for addition with string literals, eg:

operator+(std::string const& lhs, const char *rhs);

and that's why your first expression succeeds: the str object gets the ball rolling:

str + " " + "World" 
= std::string + const char* + const char*
= std::string + const char*
= std::string

to make it work use

str += " world"
// or
str += std::string(" ") + "world";
// the last one is to better explain what's going on,