Comparing characters

This is a discussion on Comparing characters within the C++ Programming forums, part of the General Programming Boards category; Do I need to do something special with character constants if my code compares a template type with them? Code: ...

  1. #1
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    Comparing characters

    Do I need to do something special with character constants if my code compares a template type with them?
    Code:
    template <typename T>
    bool isallzeros(const basic_string<T>& str)
    {
        for ( int i = 0; i < str.length(); ++i )
        {
            if ( str[i] != '0' )
                return false;
        }
    
        return true;
    }
    If T is wchar_t, will comparing str[i] to '0' still work or do I have to do something special with '0'?

  2. #2
    C++まいる!Cをこわせ! Elysia's Avatar
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    It will work fine.
    You only have to be careful with strings, but if you use basic_string, there's no need for worry at all.
    (Though, case in point, if it's wchar and you're comparing strings, you have to add L, so you can use the macro _T or TEXT to automatically append L if it's unicode.)
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    It will work fine.
    Why? A wide character constant starts with L. If str[i] is a wchar_t then shouldn't I be using L'0'? Where do I find _T or TEXT?

  4. #4
    C++まいる!Cをこわせ! Elysia's Avatar
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    Nope, a wide string begins with L, not a wide character.
    _T or TEXT is defined in Visual Studio, don't know if they're defined in other compilers. Otherwise you could try to define them yourself depending on if you're using unicode or not.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    http://msdn2.microsoft.com/en-us/library/6aw8xdf2.aspx

    It looks like wide strings and wide characters both start with L.

    Otherwise you could try to define them yourself depending on if you're using unicode or not.
    That's my problem. T is a template type and I have no way of telling if it's char or wchar_t or some other custom type but I still need to compare it with '0'.

  6. #6
    C++まいる!Cをこわせ! Elysia's Avatar
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    Code:
    	char c = 'c';
    	wchar_t w = 'w';
    Try it yourself. It will compile.

    What I was referring to is if you're using unicode in your app or not.
    And if you can't easily say, then just specialize the template function. One for regular char and one for wchar_t and one for the rest, if need be.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  7. #7
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    What about this?
    Code:
    if ( str[i] != static_cast<T>( 0 ) )

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    You mean static_cast<T>('0')?

    I don't think the original code would work if there character was a wchar_t that when casted to char becomes '0'. So use _T() or I would imagine the static_cast would work for char and wchar_t.

  9. #9
    C++まいる!Cをこわせ! Elysia's Avatar
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    You don't need to do any cast when comparing a character (unless it's an extension by MS?). Comparing strings is an entirely different matter.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

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    So use _T()
    Great, but that only works in visual C++. Doesn't C++ itself have something liek this?
    You don't need to do any cast when comparing a character (unless it's an extension by MS?).
    Why? Comparing a string is just comparing a bunch of characters. If you can compare the characters then what makes a string so different?

  11. #11
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by Banana Man View Post
    Great, but that only works in visual C++. Doesn't C++ itself have something liek this?
    No, but that define depends on if you design uinicode or multibyte apps.
    Technically, it's defined as
    Code:
    #ifdef UNICODE
    #define _T L
    #else
    #define _T
    #endif
    So it's dependant on project settings and not the type T you pass to your template function.
    If this behavior is what you want, you can define it yourself.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  12. #12
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    >> You don't need to do any cast when comparing a character (unless it's an extension by MS?). Comparing strings is an entirely different matter.

    The static_cast is potentially casting the '0' to wchar_t. Does the comparison without the cast do the same thing, or does it convert the character str[i] to a char? If it's converting str[i] to a char, then you would have a problem.

    The _T macro is a little more complicated than that I believe. It uses a parameter.

  13. #13
    C++まいる!Cをこわせ! Elysia's Avatar
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    But as I mentioned, you don't have to do L:
    Code:
    	char c = 'c';
    	wchar_t w = 'w';
    	if (c == 'c') cout << "YES! c == c!\n";
    	if (w == 'w') cout << "YES! w == w!\n";
    Both ifs evaluate as true. So unless this is an MS extension, you don't need a cast at all.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.

  14. #14
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    Quote Originally Posted by Daved View Post
    You mean static_cast<T>('0')?

    I don't think the original code would work if there character was a wchar_t that when casted to char becomes '0'. So use _T() or I would imagine the static_cast would work for char and wchar_t.
    Oops, I was mistaking '0' with '\0'.

    Quote Originally Posted by Elysia
    Technically, it's defined as

    Code:
    #ifdef UNICODE
    #define _T L
    #else
    #define _T
    #endif
    Actually, wouldn't it be more like this:
    Code:
    #ifdef UNICODE
    #define _T( x )   L ## x
    #else
    #define _T( x )   x
    #endif

  15. #15
    ZuK
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    Quote Originally Posted by Elysia View Post
    Both ifs evaluate as true. So unless this is an MS extension, you don't need a cast at all.
    Both char and wchar_t are integeral types. There is never a cast needed to compare them. At best you will get a warning if the comparison might not work as expected (e.g. signed <> unsigned ).
    Kurt

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