Casting const char * to STL string

This is a discussion on Casting const char * to STL string within the C++ Programming forums, part of the General Programming Boards category; I had never seen this before and was wondering how it worked. Code: char * szTest = "Hello World"; if((string)szTest ...

  1. #1
    Registered User valaris's Avatar
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    Casting const char * to STL string

    I had never seen this before and was wondering how it worked.

    Code:
    char * szTest = "Hello World";
    
    if((string)szTest == "Hello World")
    cout << "True";
    prints out true.

    Since casting the pointer to a string doesn't create a string object to compare, how exactly does this work?

  2. #2
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by valaris
    Since casting the pointer to a string doesn't create a string object to compare, how exactly does this work?
    It does create a temporary std::string object via the constructor that takes a const char*. Incidentally, szTest should be a const char*, not a char*.
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  3. #3
    Registered User C_ntua's Avatar
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    Why doesn't it try to find the "inner c-string" of szTest since it is supposed to be a string object? Why create a temporary object?

  4. #4
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by C_ntua
    Why doesn't it try to find the "inner c-string" of szTest since it is supposed to be a string object? Why create a temporary object?
    I am not sure what you mean. szTest is a pointer to char, not a std::string object.
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  5. #5
    Registered User C_ntua's Avatar
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    I ll try to explain with some code then. I am trying to simulate how I believe string works:
    Code:
    class myString
    {
         char* cStr;
         ...
    }
    
    bool operator==(const myString& str, const char* cstr)
    {
        if (strcmp(str.cStr, cstr) == 0)
              return true;
        else
              return false;
    }
    If you do this:
    Code:
    char * szTest = "Hello World";
    if((myString)szTest == "Hello World")
    cout << "True";
    Won't this code be executed:
    Code:
     if (strcmp(str.cStr, cstr) == 0)
    thus getting an error, since str is not a real myString, just a char* casted to a myString?

    Well, obviously I am wrong since this has been tested. But I am not understanding it completely.
    If you could compare a char* with another char* why don't overload the operator == for that occasion and require a cast to string for this to work?

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by C_ntua
    thus getting an error, since str is not a real myStringjust a char* casted to a myString?
    The cast creates a temporary, so there is a "real" myString. Besides, it is obvious that str is a "real" myString: the function signature declares it as such (or rather, it declares that str is a const reference to a myString object, but we can gloss over that).

    Quote Originally Posted by C_ntua
    If you could compare a char* with another char* why don't overload the operator == for that occasion and require a cast to string for this to work?
    Because pointer comparison merely compares addresses, not the values at those addresses. In other words, you would be testing if the C-style strings are the same, not whether they are equal. This is why strcmp() or strncmp() would be used for C-style strings.
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  7. #7
    Registered User C_ntua's Avatar
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    Hmm, yeah. Got a bit confused about how the casting would work. Thanx for the clarification

  8. #8
    Captain Crash brewbuck's Avatar
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    Quote Originally Posted by C_ntua View Post
    I ll try to explain with some code then. I am trying to simulate how I believe string works:
    How std::string works is irrelevant. That's the whole point of OOP.

    Anyway, your question is, "Why does this conversion occur when I request it explicitly?" To which I can only say... Uhhh? Err? Graaahg?
    Code:
    //try
    //{
    	if (a) do { f( b); } while(1);
    	else   do { f(!b); } while(1);
    //}

  9. #9
    Cat without Hat CornedBee's Avatar
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    When you're puzzled about casts, don't use C-style casts. Try the C++ casts and see what happens. For example, the above works as
    Code:
    static_cast<std::string>(szTest) == "Hello, World!"
    If static_cast works, then you know that nothing really weird is going on.
    All the buzzt!
    CornedBee

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