What does "local variables at the outermost scope of the function may not use the same name as any parameter" mean?

Question

I have been reading the C++ primer 5th edition. In the third paragraph of Function Parameter List of Chapter 6.1 . It writes "Moreover, local variables at the outermost scope of the function may not use the same name as any parameter". What does it mean?

I am not native English speaker. I don't understand the actual meanings of "outermost scope" of the function.

Answer 1

The outermost scope of the function is the block that defines the function's body. You can put other (inner) blocks inside that, and declare variables in those which are local to that block. Variables in inner blocks can have the same name as those in an outer block, or the function parameters; they hide the names in the outer scope. Variables in the outer block can't have the same name as a function parameter.

To demonstrate:

void f(int a)           // function has a parameter
{                       // beginning of function scope
    int b;              // OK: local variable
    {                   // beginning of inner block
        int a;          // OK: hides parameter
        int b;          // OK: hides outer variable
    }                   // end of inner block
    int a;              // Error: can't have same name as parameter
}

Answer 2

It means you can't do things like this:

void foo (int x)
{
    int x = 4; //in the outermost scope, invalid
}

You can, however, do this:

void foo (int x)
{
    { //this introduces a new scope
        int x = 4; //not in the outermost scope, valid
    }
}