Concatenate two string literals

Arthur Collé picture Arthur Collé · May 19, 2011 · Viewed 137.1k times · Source

I am reading Accelerated C++ by Koenig. He writes that "the new idea is that we can use + to concatenate a string and a string literal - or, for that matter, two strings (but not two string literals).

Fine, this makes sense I suppose. Now onto two separate exercises meant to illuminate this .

Are the following definitions valid?

const string hello = "Hello";

const string message = hello + ",world" + "!";

Now, I tried to execute the above and it worked! So I was happy.

Then I tried to do the next exercise;

const string exclam = "!";

const string message = "Hello" + ",world" + exclam;

This did not work. Now I understand it has something to do with the fact that you cannot concatenate two string literals, but I don't understand the semantic difference between why I managed to get the first example to work (isn't ",world" and "!" two string literals? Shouldn't this not have worked?) but not the second.

Answer

James McNellis picture James McNellis · May 19, 2011
const string message = "Hello" + ",world" + exclam;

The + operator has left-to-right associativity, so the equivalent parenthesized expression is:

const string message = (("Hello" + ",world") + exclam);

As you can see, the two string literals "Hello" and ",world" are "added" first, hence the error.

One of the first two strings being concatenated must be a std::string object:

const string message = string("Hello") + ",world" + exclam;

Alternatively, you can force the second + to be evaluated first by parenthesizing that part of the expression:

const string message = "Hello" + (",world" + exclam);

It makes sense that your first example (hello + ",world" + "!") works because the std::string (hello) is one of the arguments to the leftmost +. That + is evaluated, the result is a std::string object with the concatenated string, and that resulting std::string is then concatenated with the "!".


As for why you can't concatenate two string literals using +, it is because a string literal is just an array of characters (a const char [N] where N is the length of the string plus one, for the null terminator). When you use an array in most contexts, it is converted into a pointer to its initial element.

So, when you try to do "Hello" + ",world", what you're really trying to do is add two const char*s together, which isn't possible (what would it mean to add two pointers together?) and if it was it wouldn't do what you wanted it to do.


Note that you can concatenate string literals by placing them next to each other; for example, the following two are equivalent:

"Hello" ",world"
"Hello,world"

This is useful if you have a long string literal that you want to break up onto multiple lines. They have to be string literals, though: this won't work with const char* pointers or const char[N] arrays.