Conditional operator in member-initialization list

Question

Hi,

Suppose I have this class:

class foo
{
public:
    foo() { }
    foo(const std::string& s) : _s(s) { }

private:
    std::string _s;
};

Which is a member of another class:

class bar
{
public:
    bar(bool condition) : 
       _f(condition ? "go to string constructor" : **go to empty ctor**) 
    { 
    }

private:
    foo _f;
};

When initializing _f in bar's member initialization list I would like to choose which constructor of foo to invoke based on condition.

What can I put instead of go to empty ctor to make this work? I thought of putting foo(), is there another way?

Answer 1

The result of a conditional operator is always a fixed type determined at compile time by finding a common type that both options can be converted to. (The exact rules are a little involved, but in common use it usually 'does the right thing'.)

In your example the simplest thing to do is to let that type be a temporary foo and then use the copy constructor to intialize _f in bar.

You can do this as follows.

_f( condition ? foo("string") : foo() )

Answer 2

If passing an empty string is the equivalent of calling to no-arg constructor you could do the following:

_f(condition ? "string" : "")

This would save you the copy.