Necessity of declaration of function in c and cpp

Question

From bruce eckel --" although u should always declare functions by including header file , functions declarations aren't' essential in c . Its possible in c but not cpp to call a function u havent declared. This is a dangerous practise because the c compiler may assume that a function that u call with an integer argument has an argument list containing integer even if it may actually contain float . This can produce bugs" my question is that even if a function is not declared , during its definition we have to mention the data type of arguments [ VOID FUNC( INT A)] , so how can a compiler assumes a float to be an integer??

Answer 1

The compiler makes assumption on supplied parameters if a function is not declared or defined prior to the point the assumption should be made. Try the following code and check the result (checked with gcc):

#include <stdio.h>

int main (int argc, char * argv[])
{
        x(1);
        x(1.);
        x(1);
        return 0;
}

void x(double y)
{
    printf ("%f\n", y);
}