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    Jun 2001

    C++ String: How to assign or compare strings?

    Q: How to assign or compare strings?

    A: Assignment and comparison are not necessarily related, but we decided to handle them together because the mistakes made by programmers beginning with C++ have a common root, that is, the tendence to use a 'natural' syntax.


    This actually is an issue only when working with C-style strings, i.e. 'char*'. The most common mistake is this:

    char *s1, *s2;
    s2 = s1; // might not do what one would think
    This statement assigns to the char pointer 's2' the same vale as 's1'. With other words, 's1' and 's2' will point to one and the same address in memory. By no means is the string 's1' copied into 's2'.

    Another mistake that is frequently made by beginners is something like this:

    char* s = "Hello";
    s[1] = 'a';             // attempting to change the 'e' to an 'a'
    Chances are to get an access violation when running this code. The reason is that, again, the assignment applies to the char pointer 's', i.e. it will point to the address of the constant string literal "Hello'. Since the compiler is allowed to place such const literals in read-only memory, attempting to modify it has undefined results.

    The correct solution is to use the 'strcpy()' function or one of its relatives.

    char *s;
    s = new char[strlen("Hello") + 1];  // reserve one byte for the trailing '\0'
    strcpy(s, "Hello");
    delete[] s;
    Both 'CString' and the Standard C++ class 'std::string' are user-friendly and take care by themselves of this details. This code is perfectly ok:

    #include <string>
    std::string s;
    s = "Hello";
    s[1] = 'a';

    This also is an issue only when working with C-style strings, i.e. 'char*'. The most common mistake is this:
    char *s1, *s2;
    if(s1 == s2)       // wrong!!!!
    This statement may seem to be the natural syntax, but it unfortunately compares the value of two char pointers and by no means the strings they point to. To compare C-style strings you have to use the 'strcmp()' function or one of its relatives:

    char *s1, *s2;
    if(!strcmp(s1, s2))
    Note, that 'strcmp()' returns 0 if the strings are identical, thus, the '!strcmp(...)' in the if statement. Have a look in e.g. MSDN for further details on 'strcmp()' and other string comparison functions.

    Both 'CString' and the Standard C++ class 'std::string' have overloaded comparison operators, which leads to a more natural syntax:

    #include <string>
    std::string s1, s2;
    if(s1 == s2)
    All this being said, it should be clear why you always should prefere working with a string wrapper class over working with C-style strings. And there is much more std::string can do for you than this!

    Last edited by Andreas Masur; July 24th, 2005 at 06:32 AM.

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