Casting const char * to STL string

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

Code:

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char * szTest = "Hello World";

if((string)szTest == "Hello World")
cout << "True";

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prints out true.

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

Quote:

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Originally Posted by valaris

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

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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*.

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?

Quote:

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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?

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I am not sure what you mean. szTest is a pointer to char, not a std::string object.

I ll try to explain with some code then. I am trying to simulate how I believe string works:

Code:

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class myString
{
    char* cStr;
    ...
}

bool operator==(const myString& str, const char* cstr)
{
    if (strcmp(str.cStr, cstr) == 0)
          return true;
    else
          return false;
}

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If you do this:

Code:

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char * szTest = "Hello World";
if((myString)szTest == "Hello World")
cout << "True";

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Won't this code be executed:

Code:

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if (strcmp(str.cStr, cstr) == 0)

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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?

Quote:

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Originally Posted by C_ntua

thus getting an error, since str is not a real myStringjust a char* casted to a myString?

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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:

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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?

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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.

Hmm, yeah. Got a bit confused about how the casting would work. Thanx for the clarification :)

Quote:

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Originally Posted by C_ntua

I ll try to explain with some code then. I am trying to simulate how I believe string works:

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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?

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:

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static_cast<std::string>(szTest) == "Hello, World!"

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If static_cast works, then you know that nothing really weird is going on.