const string doesnt work if appended at the end

Question

Possible Duplicate:
Concatenate two string literals

Why doesn't this work ?

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

BUT this works fine

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

Please explain to me.

Thanks

http://www.cplusplus.com/forum/beginner/13507/ – csj Jun 12 '12 at 21:45 — csj, Jun 12 '12 at 21:45

Bo Persson · Answer 1 · 2012-06-12T22:24:10.873

5

The reason is that there is no operator+ for adding two string literals, and it is not needed. Your first example works if you just remove the +.

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

because ~~preprocessor~~ compiler magic*) will add the two adjacent literals together.

The second example works because std::string does have an operator+ that adds a string literal. The result is another string, that can concatenate the next literal.

*) Translation phase 6 - Adjacent string literal tokens are concatenated.

edited Jun 12 '12 at 22:24

answered Jun 12 '12 at 21:46

Bo Persson

90,663
31
146
203

nitpick, string concatenation isn't actually done by the preprocessor, (which means it doesn't have to be avoided the way we avoid the preprocessor ;) – bames53 Jun 12 '12 at 22:12
Ok, make that *compiler* magic. – Bo Persson Jun 12 '12 at 22:24

score 3 · Answer 2 · answered Jun 12 '12 at 21:43

3

Because expression "Hello" + ", world" doesn't involve any std::string, but two const char[] arguments. And there is no operator+ with that signature. You have to convert one of those to std::string first:

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

answered Jun 12 '12 at 21:43

Cat Plus Plus

125,936
27
200
224

1

This would work, too: `const std::string message = "Hello" + (", world" + exclam);` – jrok Jun 12 '12 at 21:52

score 1 · Answer 3 · answered Jun 12 '12 at 21:42

1

std::string has a + operator which is what is being used in the second example. const char * does not have that operator, which is being used in the first example.

answered Jun 12 '12 at 21:42

Anon Mail

4,660
1
18
21

score 0 · Answer 4 · answered Jun 12 '12 at 21:45

0

It's down to associativity.

The second case begins (from the left) evaluation with std::string which is concatenated with operator+. The first case begins with const char * and no concatenation operator+ exists for that.

answered Jun 12 '12 at 21:45

John

6,433
7
47
82

http://www.learncpp.com/cpp-tutorial/31-precedence-and-associativity/ – John Jun 12 '12 at 21:48

score 0 · Answer 5 · answered Jun 12 '12 at 21:49

0

The "const string doesn't work if appended at the end" is a red herring. This doesn't work, either:

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

The reason this doesn't work has been explained in the other answers.

answered Jun 12 '12 at 21:49

David Hammen

32,454
9
60
108

const string doesnt work if appended at the end

5 Answers5