How to init constructor parameters of a member in C++

I have the following code (header file):

class InnerClass
{
    InnerClass(int var);
}

class BigClass
{
    BigClass();
    InnerClass member(5);
}

How can I initialize the parameter of member in BigClass to 5 (the above code generates an error, as you probably guessed)? If I put the InnerClass member(5) line in the code (not header) file, then no problem.