Static keyword - it's multiple meanings in C++

The keyword “static” has several different meanings in C++ in different contexts. It can be confusing so this document outlines three usages you are likely to see in our code base.

Static Variables In A Function

when a variable is defined as static its value persists between function calls. The output of bar() in the following sample is 1 3. If b isn’t defined as static then the output would be 1 2.

int FOOfoo (int a) {
  static int b = 0;
  return b + a;  
}

void FOObar () {
  int c = 0;
  c = foo(1);
  COLcout << c;
  c = foo(2);
  COLcout << c;
}

Static Function Declaration

You can use static to define a function to prevent it from being exposed by the C Linker outside of the this cpp file. This is useful if you want to create a local helper function in a CPP file and want to be sure that it doesn’t clash with another symbol in the application - i.e. collide with a function of the same name in a different cpp file - reduces the symbol table sizes generated by the linker.

static void FOObar() {
  //Code to do cool stuff
}

Static Global Variable or Static Class Member

In this context using the static keyword means that there is a single instance of the variable for the entire application. The same applies to a static member of a class.

// This COLdll instance is global for the application.
static COLdll Library;

class FOObar{
public:
private:
   static COLstring m_AppName;   // Only one instance of variable for the entire application
};