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Edmond Liu 53470d2fd8 Update index.md (#36349 )

Removed some statements talking about the optimization potential of goto (which is mentioned but never specified), and I gave a possible use-case for goto followed by some constructs that can replace it, and ended on why goto is discouraged in C++

Added information about the memory behaviour of labels, the syntax to create a label, and where a label can actually be placed (within the same function).

2019-07-15 07:04:27 -07:00

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title

title
goto

Intro to the use of goto and labels

goto label goes to the mentioned occurence of the label, which can be either before or after the goto statement, as long as the label is in the same function as the goto statement.

If a goto causes program execution to exit some scope where a variable is defined, then the variable will be destroyed. If multiple of these variables exist, then they will be destroyed in opposite order of their construction.

See https://en.cppreference.com/w/cpp/language/goto for more information.

A common use of goto is to break out of a multiply-nested loop following some condition. However, there are several C++ language constructs that can be used to avoid this use case, including early returns, refactoring into different functions, and local variables in the loop.

The use of goto is discouraged in C++, since it encourages poor design and creates code that is hard to debug and trace through. https://stackoverflow.com/questions/3517726/what-is-wrong-with-using-goto#3517746)

Terminology

goto - The keyword used to go to the particular label.
label - this can be named anything.

Syntax

goto labelName; //This takes the program flow to the next appearance of label.
labelName: //to create a label name, write the name followed by a colon

Scope

goto can only jump to a label in the same scope (set of braces - {}).

Example:

#include <iostream> 
int main(){
    goto x:
    std::cout << "Hello World"; //no text is printed.
    x:
    return 0;
}
int someFunction(){
    x: //the program does not jump to this label becaue it is in another scope.
    return 0;
}

goto is something that transcends all loops. To be clearer on this point, here is an example.

#include <iostream>
using std::cout;

int main(){
    for(;;){
        if(1) goto label;
    }
    
label:
    cout << "lol";
    //here, goto is used to get out of an otherwise infinite loop. That is one of the only places where goto is tolerated.
    
    return 0;
}

Even though the above code works, a much better option is to structure your code such that goto is not needed for the program flow. For this reason, many modern programming languages (like java, javascript, python, etc.) do support goto. Instead, control statements like break and continue are used.

The above example can be rewritten using break as:

#include <iostream>
using std::cout;

int main(){
    for(;;){
        if(1) break;
    }
    
    cout << "lol";
    
    return 0;
}

However, care must be taken to use goto very carefully, especially in the early days of coding as it can lead to crazy issues, if not understood well enough. goto violates the standard flow of the program, and as C++ is an object oriented language, goto should NEVER EVER, EVER be used in a normal program, under ANY CIRCUMSTANCES. The same effect can usually be replicated by using functions or loops, with the resulting code being easier to read as well as maintain.

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