Difference between revisions of "cpp/language/goto"

Revision as of 10:33, 21 December 2020

Transfers control unconditionally.

Used when it is otherwise impossible to transfer control to the desired location using other statements.

Syntax

Explanation

The goto statement transfers control to the location specified by label. The goto statement must be in the same function as the label it is referring, it may appear before or after the label.

If transfer of control exits the scope of any automatic variables (e.g. by jumping backwards to a point before the declarations of such variables or by jumping forward out of a compound statement where the variables are scoped), the destructors are called for all variables whose scope was exited, in the order opposite to the order of their construction.

The goto statement cannot transfer control into a try-block or into a catch-clause, but can transfer control out of a try-block or a catch-clause (the rules above regarding automatic variables in scope are followed)

If transfer of control enters the scope of any automatic variables (e.g. by jumping forward over a declaration statement), the program is ill-formed (cannot be compiled), unless all variables whose scope is entered have

(Note: the same rules apply to all forms of transfer of control)

Keywords

goto

Notes

In the C programming language, the goto statement has fewer restrictions and can enter the scope of any variable other than variable-length array or variably-modified pointer.

Example

#include <iostream>
 
struct Object {
    // non-trivial destructor
    ~Object() { std::cout << "d"; }
};
 
struct Trivial {
    double d1;
    double d2;
}; // trivial ctor and dtor
 
int main()
{
    int a = 10;
 
    // loop using goto
label:
    Object obj;
    std::cout << a << " ";
    a = a - 2;
 
    if (a != 0) {
        goto label;  // jumps out of scope of obj, calls obj destructor
    }
    std::cout << '\n';
 
    // goto can be used to leave a multi-level loop easily
    for (int x = 0; x < 3; x++) {
        for (int y = 0; y < 3; y++) {
            std::cout << "(" << x << ";" << y << ") " << '\n';
            if (x + y >= 3) {
                goto endloop;
            }
        }
    }
endloop:
    std::cout << '\n';
 
    goto label2; // jumps into the scope of n and t
    int n; // no initializer
    Trivial t; // trivial ctor/dtor, no initializer
//  int x = 1; // error: has initializer
//  Object obj2; // error: non-trivial dtor
label2:
 
    {
        Object obj3;
        goto label3; // jumps forward, out of scope of obj3
    }
label3: ;
 
}

10 d8 d6 d4 d2
(0;0) 
(0;1) 
(0;2) 
(1;0) 
(1;1) 
(1;2) 
 
dd

Further Reading

The popular Edsger W. Dijkstra essay, “Goto Considered Harmful”, presents a survey of the many subtle problems the careless use of this keyword can introduce.

See also