Difference between static, auto, global and local variable in the context of c and c++

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There are two separate concepts here:

  • scope, which determines where a name can be accessed, and
  • storage duration, which determines when a variable is created and destroyed.

Local variables (pedantically, variables with block scope) are only accessible within the block of code in which they are declared:

void f() {
    int i;
    i = 1; // OK: in scope
}
void g() {
    i = 2; // Error: not in scope
}

Global variables (pedantically, variables with file scope (in C) or namespace scope (in C++)) are accessible at any point after their declaration:

int i;
void f() {
    i = 1; // OK: in scope
}
void g() {
    i = 2; // OK: still in scope
}

(In C++, the situation is more complicated since namespaces can be closed and reopened, and scopes other than the current one can be accessed, and names can also have class scope. But that’s getting very off-topic.)

Automatic variables (pedantically, variables with automatic storage duration) are local variables whose lifetime ends when execution leaves their scope, and are recreated when the scope is reentered.

for (int i = 0; i < 5; ++i) {
    int n = 0;
    printf("%d ", ++n);  // prints 1 1 1 1 1  - the previous value is lost
}

Static variables (pedantically, variables with static storage duration) have a lifetime that lasts until the end of the program. If they are local variables, then their value persists when execution leaves their scope.

for (int i = 0; i < 5; ++i) {
    static int n = 0;
    printf("%d ", ++n);  // prints 1 2 3 4 5  - the value persists
}

Note that the static keyword has various meanings apart from static storage duration. On a global variable or function, it gives it internal linkage so that it’s not accessible from other translation units; on a C++ class member, it means there’s one instance per class rather than one per object. Also, in C++ the auto keyword no longer means automatic storage duration; it now means automatic type, deduced from the variable’s initialiser.

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