break

A break will force the exiting of the present loop iteration into the next statement outside of the loop. It has no usefulness outside of a loop structure except for the switch control statement.

continue

The continue instruction is used inside loops where it will stop the current loop iteration, initiating the next one.

goto

The goto keyword is discouraged as it makes it difficult to follow the program logic, this way inducing to errors. The goto statement causes the current thread of execution to jump to the specified label.

Syntax

label:
  statement(s);

goto label;

In some rare cases, the goto statement allows to write uncluttered code, for example, when handling multiple exit points leading to the cleanup code at a function exit (and neither exception handling or object destructors are better options). Except in those rare cases, the use of unconditional jumps is a frequent symptom of a complicated design, as the presence of many levels of nested statements.

In exceptional cases, like heavy optimization, a programmer may need more control over code behavior; a goto allows the programmer to specify that execution flow jumps directly and unconditionally to a desired label. A label is the name given to a label statement elsewhere in the function.

A goto can, for example, be used to break out of two nested loops. This example breaks after replacing the first encountered non-zero element with zero.

for (int i = 0; i < 30; ++i) {
  for (int j = 0; j < 30; ++j) {
    if (a[i][j] != 0) {
       a[i][j] = 0;
       goto done;
     }
  }
}
done:
/* rest of program */

Although simple, they quickly lead to illegible and unmaintainable code.

// snarled mess of gotos

int i = 0;
  goto test_it;
body:
  a[i++] = 0;
test_it:
  if (a[i]) 
    goto body;
/* rest of program */

is much less understandable than the equivalent:

for (int i = 0; a[i]; ++i) {
  a[i] = 0;
}
/* rest of program */

Gotos are typically used in functions where performance is critical or in the output of machine-generated code (like a parser generated by yacc.)

The goto statement should almost always be avoided, but there are rare cases where it enhances the readability of code. One such case is an "error section".

Example

#include <new>
#include <iostream>

...

int *my_allocated_1 = NULL;
char *my_allocated_2 = NULL, *my_allocated_3 = NULL;
my_allocated_1 = new (std::nothrow) int[500];

if (my_allocated_1 == NULL)
{  
  std::cerr << "error in allocated_1" << std::endl;
  goto error;
}

my_allocated_2 = new (std::nothrow) char[1000];

if (my_allocated_2 == NULL)
{  
  std::cerr << "error in allocated_2" << std::endl;
  goto error;
}
    
my_allocated_3 = new (std::nothrow) char[1000];

if (my_allocated_3 == NULL)
{  
  std::cerr << "error in allocated_3" <<std::endl;
  goto error;
}
return 0;
    
error:
  delete [] my_allocated_1;
  delete [] my_allocated_2;
  delete [] my_allocated_3;
  return 1;

This construct avoids hassling with the origin of the error and is cleaner than an equivalent construct with control structures. It is thus less error prone.

abort(), exit() and atexit()

As we will see later the Standard C Library that is included in C++ also supplies some useful functions that can alter the flow control. Some will permit you to terminate the execution of a program, enabling you to set up a return value or initiate special tasks upon the termination request. You will have to jump ahead into the abort() - exit() - atexit() sections for more information.

if (Fork branching)

The if-statement allows one possible path choice depending on the specified conditions.

Syntax

if (condition)
{
  statement;
}

Semantic

First, the condition is evaluated:

• if condition is true, statement is executed before continuing with the body.
• if condition is false, the program skips statement and continues with the rest of the program.

Example

if(condition)
{
  int x; // Valid code
  for(x = 0; x < 10; ++x) // Also valid.
    {
      statement;
    }
}

flowchart from the example

Sometimes the program needs to choose one of two possible paths depending on a condition. For this we can use the if-else statement.

if (user_age < 18)
{
    std::cout << "People under the age of 18 are not allowed." << std::endl;
}
else
{
    std::cout << "Welcome to Caesar's Casino!" << std::endl;
}

Here we display a message if the user is under 18. Otherwise, we let the user in. The if part is executed only if 'user_age' is less than 18. In other cases (when 'user_age' is greater than or equal to 18), the else part is executed.

if conditional statements may be chained together to make for more complex condition branching. In this example we expand the previous example by also checking if the user is above 64 and display another message if so.

if (user_age < 18)
{
  std::cout << "People under the age of 18 are not allowed." << std::endl;
}
else if (user_age > 64)
{
  std::cout << "Welcome to Caesar's Casino! Senior Citizens get 50% off." << std::endl;
}
else
{
  std::cout << "Welcome to Caesar's Casino!" << std::endl;
}

flowchart from the example

switch (Multiple branching)

The switch statement branches based on specific integer values.

switch (integer expression) {
    case label1:
         statement(s)
         break;
    case label2:
         statement(s)
         break;
    /* ... */
    default:
         statement(s)
}

As you can see in the above scheme the case and default have a "break;" statement at the end of block. This expression will cause the program to exit from the switch, if break is not added the program will continue execute the code in other cases even when the integer expression is not equal to that case. This can be exploited in some cases as seen in the next example.

We want to separate an input from digit to other characters.

 char ch = cin.get(); //get the character
 switch (ch) {
     case '0': 
          // do nothing fall into case 1
     case '1': 
         // do nothing fall into case 2
     case '2': 
        // do nothing fall into case 3
     /* ... */
     case '8': 
        // do nothing fall into case 9
     case '9':  
          std::cout << "Digit" << endl; //print into stream out
          break;
     default:
          std::cout << "Non digit" << endl; //print into stream out
 }

In this small piece of code for each digit below '9' it will propagate through the cases until it will reach case '9' and print "digit".

If not it will go straight to the default case there it will print "Non digit"

while (Preconditional loop)

Syntax

while (''condition'') ''statement''; ''statement2'';

Semantic First, the condition is evaluated:

1. if condition is true, statement is executed and condition is evaluated again.
2. if condition is false continues with statement2

Remark: statement can be a block of code { ... } with several instructions.

What makes 'while' statements different from the 'if' is the fact that once the body (referred to as statement above) is executed, it will go back to 'while' and check the condition again. If it is true, it is executed again. In fact, it will execute as many times as it has to until the expression is false.

Example 1

#include <iostream>
using namespace std;
 
int main() 
{
  int i=0;
  while (i<10) {
    cout << "The value of i is " << i << endl;
    i++;
  }
  cout << "The final value of i is : " << i << endl;
  return 0;
}

Execution

 The value of i is 0
 The value of i is 1
 The value of i is 2
 The value of i is 3
 The value of i is 4
 The value of i is 5
 The value of i is 6
 The value of i is 7
 The value of i is 8
 The value of i is 9
 The final value of i is 10

Example 2

// validation of an input
#include <iostream>
using namespace std;
 
int main() 
{
  int a;
  bool ok=false;
  while (!ok) {
    cout << "Type an integer from 0 to 20 : ";
    cin >> a;
    ok = ((a>=0) && (a<=20));
    if (!ok) cout << "ERROR - ";
  }
  return 0;
}

Execution

 Type an integer from 0 to 20 : 30
 ERROR - Type an integer from 0 to 20 : 40
 ERROR - Type an integer from 0 to 20 : -6
 ERROR - Type an integer from 0 to 20 : 14

do-while (Postconditional loop)

Syntax

do {
  statement(s)
} while (condition);
 
statement2;

Semantic

1. statement(s) are executed.
2. condition is evaluated.
3. if condition is true goes to 1).
4. if condition is false continues with statement2

The do - while loop is similar in syntax and purpose to the while loop. The construct moves the test that continues condition of the loop to the end of the code block so that the code block is executed at least once before any evaluation.

Example

#include <iostream>

using namespace std;
 
int main() 
{
  int i=0;

  do {
    cout << "The value of i is " << i << endl;
    i++;
  } while (i<10);

  cout << "The final value of i is : " << i << endl;
  return 0;
}

Execution

The value of i is 0
The value of i is 1
The value of i is 2
The value of i is 3
The value of i is 4
The value of i is 5
The value of i is 6
The value of i is 7
The value of i is 8
The value of i is 9
The final value of i is 10

for (Preconditional and counter-controlled loop)

The for keyword is used as special case of a pre-conditional loop that supports constructors for repeating a loop only a certain number of times in the form of a step-expression that can be tested and used to set a step size (the rate of change) by incrementing or decrementing it in each loop.

Syntax

for (initialization ; condition; step-expression)
  statement(s);

The for construct is a general looping mechanism consisting of 4 parts:

1. . the initialization, which consists of 0 or more comma-delimited variable initialization statements
2. . the test-condition, which is evaluated to determine if the execution of the for loop will continue
3. . the increment, which consists of 0 or more comma-delimited statements that increment variables
4. . and the statement-list, which consists of 0 or more statements that will be executed each time the loop is executed.

The for loop is equivalent to next while loop:

 initialization
 while( condition )
 {
   statement(s);
   step-expression;
 }

Example 1

// a unbounded loop structure
for (;;)
{
  statement(s);
  if( statement(s) )
    break;
}

Example 2

// calls doSomethingWith() for 0,1,2,..9
for (int i = 0; i != 10; ++i)
{                  
  doSomethingWith(i); 
}

can be rewritten as:

// calls doSomethingWith() for 0,1,2,..9
int i = 0;
while(i != 10)
{
  doSomethingWith(i);
  ++i;
}

The for loop is a very general construct, which can run unbounded loops (Example 1) and does not need to follow the rigid iteration model enforced by similarly named constructs in a number of more formal languages. C++ (just as modern C) allows variables (Example 2) to be declared in the initialization part of the for loop, and it is often considered good form to use that ability to declare objects only when they can be initialized, and to do so in the smallest scope possible. Essentially, the for and while loops are equivalent. Most for statements can also be rewritten as while statements.

In C++11, an additional form of the for loop was added. This loops over every element in a range (usually a string or container).

Syntax

for (variable-declaration : range-expression)
  statement(s);

Example 2

std::string s = "Hello, world";
for (char c : s) 
{
  std::cout << c << ' ';
}

will print

H e l l o ,   w o r l d

.