Thread: Explicit constructor on strings

What is the return type of the string("some text") expression? Isn't it string?

The following is telling me it is not. The explicit declaration specifier is blocking the initialization.

Code:

//with the following constructor...
explicit someClass(const std::string&);

//... this definition attempt fails
someClass test = string("Rachmaninov");

The return type is a string but you don't understand how the explicit keyword works. This should work:

Code:

someClass test( string("Rachmaninov") );

Originally Posted by Nicolai M. Josuttis: The C++ Standard Library A Tutorial and Reference (Chapter 2)

By using the keyword explicit, you can prohibit a single argument constructor from defining an automatic type conversion. A typical example of the need for this feature is in a collection class in which you can pass the initial size as constructor argument. For example, you could declare a constructor that has an argument for the inital size of a stack:

Code:

class Stack
{
    explicit Stack(int size);
    ...
};

Here, the use of explicit is rather important. Without explicit this constructor would define an automatic type conversion from int to Stack. If this happens, you could assign an int to a Stack:

Code:

Stack s;

...

s = 40;

The automatic type conversion would convert the 40 to a stack with 40 elements and then assign it to s. This is probably not what was intended. By delaring the int constructor as explicit, such an assignment results in an error at compile time.

I'm not sure I was able to follow...

Originally Posted by Nicolai M. Josuttis: The C++ Standard Library A Tutorial and Reference (Chapter 2)

By using the keyword explicit, you can prohibit a single argument constructor from defining an automatic type conversion.

Being the return type for string() a string, how come there is an implicit conversion?

Originally Posted by idem

The automatic type conversion would convert the 40 to a stack with 40 elements and then assign it to s. This is probably not what was intended.

What was intended then?

Being the return type for string() a string, how come there is an implicit conversion?

There's an implicit comversion from string to someClass. The "explicit" keyword requires you to call the constructor directly and not rely on someClass(string) to be called. Read about "explicit".

Aha! Thanks both.

So basically explicit removes all chances of a copy-initialization?

It disallows implicit conversions that would require calling a constructor like aClass(int).