def args & func pointer

**~~manav~~** · 12-28-2007

Code:

#include <iostream>
using namespace std;

double p(double b, int e=2)
{
	double r = 1.0;
	while (e--) r *= b;
	return r;
}

double (*pp)(double b);

int main()
{
	pp = p;
	cout << p << " " << p(10.0) << " " << pp << " " << pp(10.0) << endl;

	return 0;
}

The above code gives me the following error on MSVS .Net 2003:
test4.cpp(15) : error C2440: '=' : cannot convert from 'double (__cdecl *)(double,int)' to 'double (__cdecl *)(double)'
This conversion requires a reinterpret_cast, a C-style cast or function-style cast

**Elysia** · 12-28-2007

This makes sense if you understand assembly.
The function p takes a double and an int (regardless if the int is optional or not), so the function pointer must match that.
Actually, function pointers do not accept default arguments (not sure about the standard, so anyone feel free to correct that), so

Code:

double (*pp)(double, int = 2);

Wouldn't work either. All arguments must be required.
Although this might work in VC7.1 (.NET 2003) and lower, it doesn't work anymore in VC8 (2005).

**~~manav~~** · 12-28-2007

Thanks Elysia. You're a programming master! How the hell do you know every thing?

**Elysia** · 12-29-2007

You're not the first who has said that

That mentioned, it's typically because I have years of experience and you learn lots around this wonderful place!

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