29.2 goto

All the sample programs in this book were coded without using a single goto. In actual practice I find I use a goto statement about once every other year. For those rare times that a goto is necessary, its syntax is:

goto label; 

where label is a statement label. Statement labels follow the same naming convention as variable names. Labeling a statement is done as follows:

label: statement; 

For example:

    for (x = 0; x < X_LIMIT; ++x) { 
        for (y = 0; y < Y_LIMIT; ++y) { 
            assert((x >= 0) && (x < X_LIMIT));
            assert((y >= 0) && (y < Y_LIMIT));
            if (data[x][y] == 0) 
                goto found; 
        } 
    } 
    std::cout << "Not found\n"; 
    exit(8); 

found: 
    std::cout << "Found at (" << x << ',' << y << ")\n"; 

One of the things you don't want to do is to use a goto statement to skip over initialization code. For example:

{
    goto skip_start;

    {
        int first = 1;

skip_start:
       printf("First is %d\n", first);
    }
}

This confuses the compiler and should be avoided.

Question 29-1: Why does Example 29-1 not print an error message when an incorrect command is entered? Hint: There is a reason I put this in the goto section.

Example 29-1. def/def.cpp

#include <iostream>
#include <cstdlib>              

int main(  )
{
    char  line[10];

    while (true) {
        std::cout << "Enter add(a), delete(d), quit(q): ";
        std::cin.getline(line, sizeof(line));

        switch (line[0]) {
        case 'a':
            std::cout << "Add\n";
            break;
        case 'd':
            std::cout << "Delete\n";
            break;
        case 'q':
            std::cout << "Quit\n";
            exit(0);
        defualt:
            std::cout << "Error:Bad command " << line[0] << '\n';
            break;
        }
    }
    return (0);
}