Why does a function declaration within another function compile and what does it do?

Question

I intended to call a private class member function, but by a copy&paste mistake pasted the line as this function is declared in the header file:

void DebugView::on_cbYAxisEnabled_stateChanged(int) {     void updateAxisEnabled(); } 

instead of

void DebugView::on_cbYAxisEnabled_stateChanged(int) {     updateAxisEnabled(); } 

Surprisingly, the code was compiled and executed. However the method updateAxisEnabled() was not executed.

So, why does it compile? Was here a local function declared within a method body or has void instructed the compiler to ignore whatever comes afterwards?

The compiler is Visual Studio 2008.

P.S.: I'm aware of class declaration/definition within functions, but not functions within functions in C++.

Answer 1

void updateAxisEnabled(); is a function declaration.

Sample:

#include <cstdio>  void a(); void b();  int main(void) {     a();     b();     return 0; }  void a() {     void c(); // Declaration     c(); // Call it }  void b() {     c(); // Error: not declared }  void c() {     puts("Hello, world!"); } 

Answer 2

It is perfectly allowed to declare a function inside a function scope: a function may be declared in any scope.

A common mistake among C++ programmers is indeed to:

void foo() {     MyObject bar(); // 1     bar.someMethod(); // 2 } 

this will miserably fail to compile because line 1 is not declaring a MyObject named bar and calling its constructor explicitly; rather, it is declaring a function named bar that returns a MyObject. Thefore, there is really no object to call someMethod on.