Re: Question on adding strings

From:

"Abhishek Padmanabh" <abhishek.padmanabh@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 31 Mar 2008 22:12:54 +0530

Message-ID:

<fsr4ag$9fv$1@aioe.org>

"C++ Newbie" <newbie.cpp@googlemail.com>

Consider the following program from "Accelerated C++", Koenig & Moo
[2004]:

Case [1]:
=====================================
#include <iostream>
#include <string>

int main()
{

// Ex 1-2
       const std::string exclam = "!";
       const std::string message2 = "Hello" + ", world" + exclam ;
       std::cout << message2;
       std::cout << std::endl;
return 0;
}
=====================================

The above fails to compile with the error:
1-3.cpp: In function 'int main()':
1-3.cpp:18: error: invalid operands of types 'const char [6]' and
'const char [8]' to binary 'operator+'

(Line numbers are a bit off as I snipped out irrelevant code)

If we move around the string addition a bit to a less sensible
arrangement:
Case 2:
=======================================
#include <iostream>
#include <string>

int main()
{

// Ex 1-1
       const std::string hello = "Hello";
       std::cout << hello;
       std::cout << std::endl;

       const std:: string message = hello + ", world" + "!";
       std::cout << message;
       std::cout << std::endl;

// Ex 1-2
       const std::string exclam = "!";
       const std::string message2 = "Hello" + exclam + ", world" ;
       std::cout << message2;
       std::cout << std::endl;

return 0;
}
====================================

The above compiles and runs.

Why does changing the arrangement of message2 make such a big
difference?

The difference here is caused by the rules for operator associativity. The
operator in question here is operator+ that, I think, has left-to-right
associativity. So, for the first case, when the evaluation starts, it picks
up two the two string literals that don't have the operator+ defined for
them. For the other two cases, one argument is std::string that has two
operator+'s defined that returns back another std::string which then gets
evaluated with the right-most argument and it works its way out.

std::string operator+(const char* str1, const std::string& str2);

and

std::string operator+(const std::string& str1, const char* str2);