How to Returning Values in C and C++
Returning Values
All functions, except those of type void, return a value. This value is specified by the return statement. In C-language, if a non-void function does not explicitly return a value via a return statement, then a garbage value is returned. In C++, a non-void function must contain a return statement that returns a value. That is, in C++, if a function is specified as returning a value, any return statement within it must have a value associated with it. However, if execution reaches the end of a non-void function, then a garbage value is returned. Although this condition is not a syntax error, it is still a
fundamental error and should be avoided.
As long as a function is not declared as void, you may use it as an operand in an expression. Therefore, each of the following expressions is valid:
x = power(y);
if(max(x,y) > 100) printf(”greater”);
for(ch=getchar(); isdigit(ch); ) … ;
As a general rule, a function cannot be the target of an assignment. A statement such as
swap(x,y) = 100; /* incorrect statement */
is wrong. The C/C++ compiler will flag it as an error and will not compile a program that contains it. (As is discussed in Part Two, C++ allows some interesting exceptions to this general rule, enabling some types of functions to occur on the left side of an assignment.)
When you write programs, your functions generally will be of three types. The first type is simply computational. These functions are specifically designed to perform operations on their arguments and return a value based on that operation. A computational function is a “pure” function. Examples are the standard library functions sqrt() and sin(), which compute the square root and sine of their arguments.
The second type of function manipulates information and returns a value that simply indicates the success or failure of that manipulation. An example is the library function fclose(), which is used to close a file. If the close operation is successful, the function returns 0; if the operation is unsuccessful, it returns EOF.
The last type of function has no explicit return value. In essence, the function is strictly procedural and produces no value. An example is exit(), which terminates a program. All functions that do not return values should be declared as returning type void. By declaring a function as void, you keep it from being used in an expression, thus preventing accidental misuse.
Sometimes, functions that really don’t produce an interesting result return something anyway. For example, printf() returns the number of characters written. Yet it would be unusual to find a program that actually checked this. In other words, although all functions, except those of type void, return values, you don’t have to use the return value for anything. A common question concerning function return values is, “Don’t I have to assign this value to some variable since a value is being returned?” The answer is no. If there is no assignment specified, the return value is simply discarded. Consider the following program, which uses the function mul():
#include
int mul(int a, int b);
int main(void)
{
int x, y, z;
x = 10; y = 20;
z = mul(x, y); /* 1 */
printf(”%d”, mul(x,y)); /* 2 */
mul(x, y); /* 3 */
return 0;
}
int mul(int a, int b)
{
return a*b;
}
In line 1, the return value of mul() is assigned to z. In line 2, the return value is not actually assigned, but it is used by the printf() function. Finally, in line 3, the return value is lost because it is neither assigned to another variable nor used as part of an expression.
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