The assert macro in C++ is defined conditionally based on the presence of the NDEBUG macro. In release builds, where NDEBUG is defined, it expands to ((void)0).

#ifdef NDEBUG
    #define assert(condition) ((void)0)
#else
    // Debug-mode implementation
#endif

The expressino ((void)0) involves an explicit cast of the integer literal 0 to the void type. The C++ standard permits any expression to be cast to void, which siginfies the absence of a value. The outer parentheses ensure safe macro expansion.

The standard requires the assert macro to produce no value. This prevents incorrect usage, such as:

// These would cause errors
// bool flag = assert(ptr != nullptr);
// process(assert(x > 10));

Defining assert as an empty macro (e.g., #define assert(condition)) leads to issues in comma expressions:

int result = 0;
result = compute_value(), assert(result != 0);

In a release build, an empty macro would expand to:

result = compute_value(), ;

This is invalid syntax. Using ((void)0) expands to:

result = compute_value(), (void)0;

This is syntactically correct. The (void)0 acts as a harmless statement, preserving the expression's validity.

Thus, ((void)0) is used because it forms a valid, side-effect-free expression of type void, meeting the standard's requirement for a no-op that maintains syntactic integrity.