Can't add strings in C++

Because in C++, string literals (like "Hello" are not of type std::string. They are plain char arrays, or C-style strings.

So for the line const std::string message = "Hello" + ", world" + exclam;,the types the compiler has to work with are:

const std::string message = const char[6] + const char[8] + std::string;

and given the associativity of +, the operations it has to perform are:

const std::string message = ((const char[6] + const char[8]) + std::string);

That is, the left-most addition must be evaluated first, and the result passed to the rightmost addition.

So the compiler tries to evaluate const char[6] + const char[8]. There is no addition defined for arrays. Arrays are implicitly converted to pointers, but this doesn't help the compiler. That just means it ends up with const char* + const char*, and no addition is defined for pointers either.

At this point, it doesn't know that you want the result to be converted to a std::string.

However, in your second example:

const std::string hello = "Hello";
const std::string message = hello + ", world" + "!";

it works, because the operations the compiler would see were std::string + const char[8] + const char[2]. Here, the first addition can be converted to std::string + const char*, and here the addition operator is defined, and returns a std::string. So the compiler has successfully figured out the first addition, and since the result was a string, the second addition looks like this: std::string + const char[2], and like before, this isn't possible, but the array can be converted to a pointer, and then the compiler is able to find an addition operator that works, again resulting in a std::string.

"Hello" + ", world"

Since these are c-style strings, you cannot append them with +. You can append a std::string to a c-style string, but not 2 c-style strings this way, instead add a std::string() constructor around one of them to make a temporary, ie:

"Hello" + std::string(", world")

C++ doesn't do many of the automatic 'behind the scenes' conversations of other OO languages.

As Doug said you need to do std::string("hello") + std::string(" world"), the language doesn't do this for you.

However you can do

std::cout << "hello" << "world" << exclam;

Because std::cout knows how to print a const char[] as well as a string