Undeclared Identifier Error when using function returning pointers to functions (C++ on Visual Studio)

Question

I wrote the following function:

typedef float (*flIntPtr)(char);
flIntPtr c(int i) {
    flIntPtr pt = &b;
    return pt;
}

float b(char c) {
....//do something
}

Then visual studio reported that b is an undeclared identifier. I have go through the examples of possible causes to C2065:undeclared identifier below: Compiler Error C2065

To avoid typo or similar problem, I used single letters as function names, and so on. I also went through similiar questions provided by stackoverflow, there I noticed a similar problem hence I thought it might be caused by mistakenly written expression of the function pointer or type mismatch, since I didn't think my typedef part is wrong, I tried to change flIntPtr pt = &b; to flIntPtr pt = b ; and flIntPtr pt = *b; but the error remained. Thus, I am once again asking for your technical support.~

Answer 1

Your compiler tries to understand your code by digesting it from the top to bottom. This means that you need to have a inverted tree like structure (meaning that you need to have all the dependencies of a dependent somewhere on the top of it). When you don't have it, the compiler will flag an error.

int main()
{
    foo();    // <-- Here the compiler doesn't know that a function called foo is there somewhere so it'll flag an error.
    return 0;
}

void foo() {}

For this, what you need is Forward Declarations . These basically tell the compiler "Hey man, there's a function named 'foo' somewhere. Don't flag an error and be a complete moron let the linker handle it.". It'll look something like this,

void foo();    // <-- This is the forward declaration.
int main()
{
    foo();    // <-- Now the compiler will leave it alone for the linker to resolve this.
    return 0;
}

void foo() {}

This is not only for functions, its the same for classes and structs too. These are specially useful when it comes to circular dependencies .