What happens if the main function is declared without a return type?

When the main function is declared without a return type in C or C++, that leads to a violation of the language standards. The main function must have an explicit return type, which in these languages is typically int. The reason for requiring an explicit return type is to ensure that the program can properly communicate its exit status to the operating system.

When the function does not specify a return type, the compiler cannot determine how to process the function call, leading to an error in compilation. This is critical because the return type specifies the kind of data that can be returned, and in the case of the main function, it indicates whether the program execution was successful (returns 0) or failed (returns a non-zero value). Therefore, a declaration without a return type results in compilation failure, as the compiler requires strict adherence to syntax rules for successful code compilation.