Can we use function on left side of an expression in C and C++?

In C and C++, you normally cannot use a function call on the left side of an assignment if it returns a value by copy, because function calls return non-assignable temporary values. However, there are specific cases where this is possible −

Syntax

// Invalid - function returns value by copy
function_name() = value;        // Compiler error

// Valid - function returns pointer (C)
*function_name() = value;       // Dereference pointer

// Valid - function returns reference (C++ only)
function_name() = value;        // Direct assignment to reference

Understanding lvalues and rvalues

To use something on the left side of an assignment, it must be an lvalue − a modifiable memory location. Function calls that return values by copy are rvalues (temporary values) and cannot be assigned to.

Example 1: Valid Case in C - Using Pointer Dereference

This example shows how to use a function on the left side by returning a pointer and dereferencing it −

#include <stdio.h>

// Function returns a pointer to a static variable
int* getPtr() {
    static int x = 5;
    return &x;
}

int main() {
    printf("Before: %d\n", *getPtr());
    
    // Valid: Assigning through pointer dereference
    *getPtr() = 100;
    
    printf("After: %d\n", *getPtr());
    return 0;
}

Before: 5
After: 100

Example 2: Invalid Case - Function Returns Value by Copy

This example demonstrates what happens when you try to assign to a function that returns a value −

#include <stdio.h>

// Function returns a value (not a pointer)
int getValue() {
    return 5;
}

int main() {
    // This will cause a compiler error
    // getValue() = 10;  // Uncomment to see error
    
    printf("Function returned: %d\n", getValue());
    printf("Cannot assign to function return value\n");
    return 0;
}

Function returned: 5
Cannot assign to function return value

Key Points

• Functions returning values by copy cannot be used on the left side of assignment
• Functions returning pointers can be dereferenced and assigned to: *function() = value
• In C++, functions returning references can be directly assigned to: function() = value
• Always ensure returned pointers/references point to valid memory (use static variables, not local ones)

Conclusion

Functions can only be used on the left side of expressions in C when they return pointers that can be dereferenced. This technique is useful for creating modifiable access to static or global variables through function interfaces.