Beginner question: string("Chimera")

Hello all,

I am just starting out on C++ having spent some time learning C.

I have come across the following declarations in Chapter 6 of Lippman (3rd edition):

Code:

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extern int get_word_count(string file_name);
vector<string> svec(get_word_count(string("Chimera")));

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I am rather puzzled by the expression string("Chimera").

Is this casting a string literal into a string type or something? I haven't been able to find any reference to such expressions anywhere. The closest I have found is declarations of the type string string_name("Chimera"), but in the example above there is no identifier.

Also, what value would in such an expression - or the parameter string file_name - have (in C it would be a pointer to char; is it the same in C++, or is the value of an object simply the object itself?)?

Many thanks for any help.

It creates a temporary string object using the constructor that takes const char*, to pass it to a function.

Thanks. Can you do that other (all?) constructors? E.g. would the following work?

Code:

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string s1("Hello!");
extern int get_word_count(string file_name);
vector<string> svec(get_word_count(string(s1)));

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Quote:

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E.g. would the following work?

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Before asking, try it yourself.

That said, yes, it would work, but you might as well write:

Code:

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vector<string> svec(get_word_count(s1));

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That would use the copy constructor. In this case there is no actual need for a copy, so you might just use get_word_count(s1).

And yes, it would work for all kinds of constructors:

Code:

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    string s("hello");
    get_word_count(string()); //default constructor (empty string)
    get_word_count(string(s)); //using copy constructor
    get_word_count(string("Hello")); //from const char*
    get_word_count(string("hello", 3)); //first three characters of "hello"
    get_word_count(string(5, 'a')); //= "aaaaa"
    get_word_count(string(s.begin(), s.begin() + 2)); //from range = "he"
    get_word_count(string(s, 1, 3)); //substring of s = "elo"

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Quote:

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Originally Posted by DL1

Thanks. Can you do that other (all?) constructors? E.g. would the following work?

Code:

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string s1("Hello!");
extern int get_word_count(string file_name);
vector<string> svec(get_word_count(string(s1)));

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Yes, that should work.

You can do it with all constructors, even primatives. For primatives it acts just like a C style cast.

It simply calls a constructor for the specified type.
It can be done with any type, providing it has an appropriate constructor.
Since you do not assign it to anything, the object becomes temporary - it usually dies after the line is question.

Thanks again.

And one last option that isn't mentioned:

Code:

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extern int get_word_count(string file_name);
vector<string> svec(get_word_count("Chimera"));

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When you pass const char* to a function that expects a string, the string file_name will be constructed using implicitly the suitable constructor.