29.2 goto

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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); } 

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