C++Talk.NET

Maciej Sobczak · Guest

Hi,

Consider this code:

#include <iostream>

using namespace std;

void do_int(int &i)
{
cout << "computing int resultn";
i = 7;
}

void do_double(double &d)
{
cout << "computing double resultn";
d = 3.1415926;
}

struct ReturnDispatch
{
operator int() const
{
int res;
do_int(res);
return res;
}

operator double() const
{
double res;
do_double(res);
return res;
}
};

ReturnDispatch foo()
{
return ReturnDispatch();
}

int main()
{
int i = foo(); // (1)
cout << i << 'n';

double d = foo(); // (2)
cout << d << 'n';
}

An interesting feature is that when the return value from foo() is
ignored, it is not computed at all. For some it may be a bug, but then
it is possible to instrument the destructor of ReturnDispatch to do
something relevant in the given context.

I would like to know whether this idiom is in use and what is your
experience with it.

--
Maciej Sobczak : http://www.msobczak.com/
Programming : http://www.msobczak.com/prog/

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Chris Theis · Guest

"Maciej Sobczak" <no_spam (AT) no_spam (DOT) com> wrote

Maciej Sobczak · Guest

What you got previously is unnecessarily complex.
The whole idiom may be shorter:

int do_int()
{
cout << "computing int resultn";
return 7;
}

double do_double()
{
cout << "computing double resultn";
return 3.14159265;
}

struct ReturnDispatch
{
operator int() const { return do_int(); }
operator double() const { return do_double(); }
};

ReturnDispatch foo() { return ReturnDispatch(); }

int main()
{
int i = foo();
cout << i << 'n';

double d = foo();
cout << d << 'n';
}

or even with templates:

#include
using namespace std;

template <typename T> T compute();

template <>
int compute()
{
cout << "computing int resultn";
return 7;
}

template <>
double compute()
{
cout << "computing double resultn";
return 3.14159265;
}

struct ReturnDispatch
{
template operator T() const { return compute<T>(); }
};

ReturnDispatch foo() { return ReturnDispatch(); }

int main()
{
int i = foo();
cout << i << 'n';

double d = foo();
cout << d << 'n';
}

Anyway - my question is still valid:

Craig Henderson · Guest

Maciej Sobczak <no_spam (AT) no_spam (DOT) com> wrote