C++ compile time checker using templates

Question

I am having following code which is taken from modern C++ design. While i am using it i am getting compiation error i think invalid sizeof opearand. Can any one point out what is the problem. Thanks!

template<bool>
struct CompileTimeChecker {
    CompileTimeChecker(...);
};

template<>
struct CompileTimeChecker<false> {
};

#define STATIC_CHECK(expr, msg) \
{\
class ERROR_##msg {}; \
(void)sizeof(CompileTimeChecker<(expr) != 0>((ERROR_##msg())));\
}

template <class To, class From>
To safe_reinterpret_cast(From from) {
    STATIC_CHECK(sizeof(From) <= sizeof(To), Destination_Type_Too_Narrow);
    return reinterpret_cast<To>(from);
}

int main(void)
{
    int a[20];
    void* somePointer = a;
    char c = safe_reinterpret_cast<int>(somePointer);
}

Error:

d:\technical\c++study\readparsing\readparsing\addressconv.cpp(29) : error C2066: cast to function type is illegal 1> d:\technical\c++study\readparsing\readparsing\addressconv.cpp(37) : see reference to function template instantiation 'To safe_reinterpret_cast(From)' being compiled 1> with 1> [ 1> To=int, 1> From=void * 1> ] 1>d:\technical\c++study\readparsing\readparsing\addressconv.cpp(29) : error C2070: 'CompileTimeChecker<__formal> (safe_reinterpret_cast::ERROR_Destination_Type_Too_Narrow (__cdecl *)(void))': illegal sizeof operand 1> with 1> [ 1> __formal=true 1> ]

I'm guessing this is a 64-bit compile, on a typical machine? If so, `void*` will likely be 8 bytes while `int` will be 4, setting off the assert. — GManNickG, Feb 27 '11 at 10:38

score 4 · Answer 1 · answered Feb 27 '11 at 10:46

4

Yet another strike for the most vexing parse...

sizeof(CompileTimeChecker<(expr) != 0>((ERROR_##msg()))

Is the same as

class Foo {};
class Bar {};
sizeof(Foo((Var()));

and as Foo(Var) can be interpreted either as a type (function taking a (function without an argument an returning a Var) and returning a Foo), it is so.

answered Feb 27 '11 at 10:46

AProgrammer

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how this is related to question, can u pls elobarate it – venkysmarty Feb 27 '11 at 10:58
This explain the "illegal sizeof operand" errors. sizeof can take a type as argument but not a function type. And that's what you are giving it mistakenly. – AProgrammer Feb 27 '11 at 11:12
`Foo((int()))` is not a function type. There is a pair of parens around the "parameter" which makes it not a parameter declaration. – Johannes Schaub - litb Feb 27 '11 at 11:18
Ah nvm it's a binding parenthesis. I'm mistaken :) – Johannes Schaub - litb Feb 27 '11 at 11:30

score 1 · Accepted Answer · answered Feb 27 '11 at 11:50

Like AProgrammer pointed out, the (void)sizeof isn't getting swallowed by the compiler. I suggest removing the parentheses from the sizeof, like this:

(void)sizeof CompileTimeChecker<(expr) != 0>((ERROR_##msg()));\

That seems to make g++ accept it, and interpret it the way it was probably meant to.

If that (void)sizeof keeps giving you trouble, you can get the static checking functionality without it too, for example by initializing a CompileTimeChecker variable:

CompileTimeChecker<(expr) != 0> a((ERROR_##msg()));\

C++ compile time checker using templates

2 Answers2