Difference between local scope and function scope

问题:

Once I assumed that these two have the same meaning but after reading more about it i'm still not clear about the difference. Doesn't the local scope sometimes refer to scope of function? and what does it mean that only labels have a function scope?

回答1:

void doSomething()
{                                    <-------
     {                   <----               | 
                              |              |
         int a;           Local Scope    Function Scope
                              |              |
     }                   <----               | 
}                                    <------- 

Function Scope is between outer { }.

Local scope is between inner { }

Note that, any scope created by {``} can be called as the local scope while the {``} at the beginning of the function body create the Function scope.
So, Sometimes a Local Scope can be same as Function Scope.

what does it mean that only labels have a function scope?

Labels are nothing but identifiers followed by a colon. Labeled statements are used as targets for goto statements. Labels can be used anywhere in the function in which they appear, but cannot be referenced outside the function body. Hence they are said to have Function Scope.

Code Example:

int doSomething(int x, int y, int z)
{
     label:  x += (y + z);   /*  label has function scope*/
     if (x > 1) 
         goto label;
}

int doSomethingMore(int a, int b, int c)
{
     if (a > 1) 
         goto label; /*  illegal jump to undefined label */
}

回答2:

Local scope is the area between an { and it's closing }. Function scope is the area between the opening { of a function and its closing }, which may contain more "local" scopes. A label is visible in the entirety of the function within which it is defined, e.g.

int f( int a ) 
{
    int b = 8;
    if ( a > 14 )
    {
       int c = 50;
       label:
       return c - a - b;
    }
    if ( a > 7 ) goto label;
    return -99;
}

int c is not visible outside its enclosing block. label is visible outside its enclosing block, but only to function scope.

回答3:

Doesn't the local scope sometimes refer to scope of function?

Yes. In most C-derived languages, variables are valid in the scope in which they're declared. If you declare a variable inside a function, but not within any other code block, then that variable is usually called a "local" or "automatic" variable. You can refer to it anywhere in the function. On the other hand, if you declare your variable inside another code block -- say, in the body of a conditional statement, then the variable is valid only inside that block. Several other answers here give good examples.

and what does it mean that only labels have a function scope?

Context would be helpful, but it means that you can't jump from one function to a label in a different function.

void foo(int a) {
    if (a == 0) goto here;  // okay -- 'here' is inside this function
    printf("a is not zero\n");
    goto there;             // not okay -- 'there' is not inside this function
here:
    return;
}

void bar(int b) {
    if (b == 0) goto there; // okay -- 'there' is in this function
    printf("b is not zero\n");
there:
    return;
}

Not to stir up a hornet's nest, but the scope of labels probably won't come up too often. Labels are mainly useful with the goto statement, which is needed only very rarely if ever, and even if you did choose to use goto you probably wouldn't even think of trying to jump into a different function.

回答4:

The scope of the function is slightly larger than the scope of the function body: The function arguments are in the outer scope, while local variables are only in the inner one. This is most visibly manifest in a function-try-block:

void f(int a) try {
  // function body
} catch(...) {
  // catch block
}

Inside the catch block, only the variables in function scope are still in scope, but not the local variables.

Of course you can and do also introduce further, deeper nested scopes all the time, e.g. in for loop bodies or conditional bodies.

回答5:

bool m[3][3];
void f1()
{
  int i;
  // redefining a variable with the same name in the same scope isn't possible
  //int i; //error C2086: 'int i' : redefinition
}

void f2()
{
  int i; // ok, same name as the i in f1(), different function scope.

  {
    int i; // ok, same name as the i above, but different local scope.
  }

  // the scope if the following i is local to the for loop, so it's ok, too.
  for (int i = 0; i < 3; i++)
  {
    for (int j = 0; j < 3; j++)
    {
      if (m[i][j])
        goto loopExit;
    }
  }
loopExit:
  std::cout << "done checking m";
// redefining a label with the same name in the same function isn't possible
// loopExit:; // error C2045: 'loopExit' : label redefined
}

void f3()
{
loopExit:; // ok, same label name as in f2(), but different function scope
}