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c++debuggingcastingassert
Elpezmuerto
Elpezmuerto

Reputation: 5571

Can you use assert to test type defintions in C++?

Can I use assert to enforce type definitions. Suppose there is a variable, double d, how can you use assert to assert that d is a double? If assert is not applicable (which I am betting isn't), is there another option? I am specifically looking to test for implicit type casting during debugging, while benefiting from the functionality of assert and #define NDEBUG.

P.S Obviously I would want to use this for any type definition, just using double as an example here. The solution should be cross platform compatible and be compatible with C++03.

I like to add error checking to my class setters. For example, suppose there is a class, MyClass, with a private member variable, x:

void MyClass::setX(double input)
{
   // assert x is double
   x = input;
}

Upvotes: 6

Views: 11664

Answers (5)

JohnPS
JohnPS

Reputation: 2616

You can create a template function, then overload the argument type for double like this:

#include <cassert>

template<class T>
bool is_double(T) { return false; }
bool is_double(double) { return true; }

int main() {
    int i = 1;
    double d = 3.14;
    assert( is_double(d) );
    assert( is_double(i) ); // fails
}

That would give a run-time error. You can generate a compile time error by simply defining a function that takes a double reference:

void is_double(double&) { }

void MyClass::setX(double input)
{
  is_double(x); // assert x is double
  x = input;
}

Upvotes: 0

K-ballo
K-ballo

Reputation: 81349

Given the current definition of the code, a way to check at compile time whether both are of the same type is:

template< typename T, typename U >
void assert_same_type( T const&, U const& )
{
     int error[ sizeof( T ) ? -1 : -2 ]; // error array of negative size, dependent on T otherwise some implementations may cause an early error message even when they shouldn't
}

template< typename T >
void assert_same_type( T&, T& ){}

void MyClass::setX(double input)
{
   assert_same_type( x, input ); // If the fallback case is instantiated then a compile time error will arise of trying to declare an array of negative size.

   x = input;
}

Upvotes: 1

Elpezmuerto
Elpezmuerto

Reputation: 5571

You can use the == operator defined in the type_info class to test for a specific type definition.

#include <assert.h>
#include <iostream>
#include <typeinfo>

int main ()
{
    double a = 0;

    std::cout << typeid(a).name() << std::endl;

    assert(typeid(a)==typeid(double));
    assert(typeid(a)==typeid(int)); // FAIL
}

Or borrowing from another SO answer using templates and try/catch:

#include <assert.h>
#include <iostream>
#include <typeinfo>

template <typename X, typename A>
inline void Assert(A assertion)
{
    if( !assertion ) throw X();
}

#ifdef NDEBUG
    const bool CHECK_ASSERT = false;
#else
    const bool CHECK_ASSERT = true;
#endif

struct Wrong { };

int main ()
{
    double a = 0;

    std::cout << typeid(a).name() << std::endl;

    assert(typeid(a)==typeid(double));
    Assert<Wrong>(!CHECK_ASSERT || typeid(a)==typeid(double));
    try
    {
    //assert(typeid(a)==typeid(int)); // FAIL and Abort()
        Assert<Wrong>(!CHECK_ASSERT || typeid(a)==typeid(int)); // FALL
    }
    catch (Wrong)
    {
        std::cerr <<"Exception, not an int" <<std::endl;
    }
}

Upvotes: 6

rubenvb
rubenvb

Reputation: 76519

You should be able to compare using std::is_same and using decltype. You can even use std::static_assert to move the check to compile time. I've seen it happen in libc++ :)

Note these are C++11 features, so you'll need to have a compiler that supports decltype

Upvotes: 3

Tom Kerr
Tom Kerr

Reputation: 10720

It's really a compile time check, so you should use static asserts for this.

Here is an example using boost's static asserts and type traits.

#include <boost/static_assert.hpp>
#include <boost/type_traits.hpp>

template<typename T>
  void some_func() {
    BOOST_STATIC_ASSERT( (boost::is_same<double, T>::value) );
  }

TEST(type_check) {
  some_func<double>();
}

I assume you mean in terms of a template anyway.

Upvotes: 11

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