Static and Automatic Variables

• Automatic variable: memory is allocated at block entry and deallocated at block exit

  • In C++, a block is statements within curly brackets (functions, loops, etc.)

  • By default, variables declared within a block are automatic variables

• Static variable: memory remains allocated if the program executes

  • Global variables declared outside of any block are static variables

  • Syntax: static dataType identifier = value;

  • Static variables declared within a block are local to the block have the same scope as any other local identifier in that block.

Example use of a static variable

double runningTotal(double numberToAdd)
{
    static double total = 0; // declare and initialize variable that
                             // remains over the whole program run.

    total += numberToAdd; // add number to total

    return total; // return updated total
}

double num = 0;
num = runningTotal(1); // num = 1.0
num = runningTotal(2); // num = 3.0
num = runningTotal(2); // num = 5.0

Question: What would happen if the variable was not static?
Answer: num would always equal the argument passed into the function:

num = runningTotal(1); // num = 1.0
num = runningTotal(2); // num = 2.0
num = runningTotal(2); // num = 2.0