Difference between revisions of "cpp/language/goto"
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{{cpp/language/statements/navbar}} | {{cpp/language/statements/navbar}} | ||
| − | Transfers control | + | Transfers control unconditionally. |
| − | Used when it is otherwise impossible to transfer control to the desired location using | + | Used when it is otherwise impossible to transfer control to the desired location using other statements. |
===Syntax=== | ===Syntax=== | ||
| Line 14: | Line 14: | ||
===Explanation=== | ===Explanation=== | ||
| − | The goto statement transfers control to the location specified by {{sparam|label}}. The goto statement must be in the same function as the {{sparam|label}} it is referring. If | + | The goto statement transfers control to the location specified by {{sparam|label}}. The goto statement must be in the same function as the {{sparam|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. | ||
| + | |||
| + | 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 | ||
| + | @1@ scalar types declared without initializers | ||
| + | @2@ class types with trivial default constructors and trivial destructors declared without initializers | ||
| + | @3@ cv-qualified versions of one of the above | ||
| + | @4@ arrays of one of the above | ||
| + | |||
| + | (Note: the same rules apply to all forms of transfer of control) | ||
| + | |||
===Keywords=== | ===Keywords=== | ||
| Line 26: | Line 36: | ||
struct Object { | struct Object { | ||
| + | // non-trivial destructor | ||
~Object() { std::cout << "d"; } | ~Object() { std::cout << "d"; } | ||
}; | }; | ||
| + | |||
| + | struct Trivial { | ||
| + | double d1; | ||
| + | double d2; | ||
| + | }; // trivial ctor and dtor | ||
int main() | int main() | ||
| Line 33: | Line 49: | ||
int a = 10; | int a = 10; | ||
| − | //loop using goto | + | // loop using goto |
label: | label: | ||
Object obj; | Object obj; | ||
| Line 40: | Line 56: | ||
if (a != 0) { | if (a != 0) { | ||
| − | goto label; // | + | goto label; // jumps out of scope of obj, calls obj destructor |
} | } | ||
std::cout << '\n'; | std::cout << '\n'; | ||
| − | // | + | // goto can be used to leave a multi-level loop easily |
for (int x = 0; x < 3; x++) { | for (int x = 0; x < 3; x++) { | ||
for (int y = 0; y < 3; y++) { | for (int y = 0; y < 3; y++) { | ||
| Line 56: | Line 72: | ||
std::cout << '\n'; | 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: ; | ||
| + | |||
} | } | ||
| output= | | output= | ||
10 d8 d6 d4 d2 | 10 d8 d6 d4 d2 | ||
| − | (0;0) (0;1) (0;2) (1;0) (1;1) (1;2) | + | (0;0) |
| − | + | (0;1) | |
| + | (0;2) | ||
| + | (1;0) | ||
| + | (1;1) | ||
| + | (1;2) | ||
| + | |||
| + | dd | ||
}} | }} | ||
Revision as of 13:46, 1 May 2013
Transfers control unconditionally.
Used when it is otherwise impossible to transfer control to the desired location using other statements.
Contents |
Syntax
Template:sparam(optional) goto Template:sparam ;
|
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Explanation
The goto statement transfers control to the location specified by Template:sparam. The goto statement must be in the same function as the Template:sparam 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.
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
1) scalar types declared without initializers
2) class types with trivial default constructors and trivial destructors declared without initializers
3) cv-qualified versions of one of the above
4) arrays of one of the above
(Note: the same rules apply to all forms of transfer of control)
Keywords
Example
Run this code
#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: ; }
Output:
10 d8 d6 d4 d2 (0;0) (0;1) (0;2) (1;0) (1;1) (1;2) dd
