I am looking for a way to identify primitives types in a template class definition.
I mean, having this class:
template<class T>
class A{
void doWork(){
if(T isPrimitiveType())
doSomething();
else
doSomethingElse();
}
private:
T *t;
};
Is there is any way to "implement" isPrimitiveType().
UPDATE : Since C++11, use the is_fundamental
template from the standard library:
#include <type_traits>
template<class T>
void test() {
if (std::is_fundamental<T>::value) {
// ...
} else {
// ...
}
}
// Generic: Not primitive
template<class T>
bool isPrimitiveType() {
return false;
}
// Now, you have to create specializations for **all** primitive types
template<>
bool isPrimitiveType<int>() {
return true;
}
// TODO: bool, double, char, ....
// Usage:
template<class T>
void test() {
if (isPrimitiveType<T>()) {
std::cout << "Primitive" << std::endl;
} else {
std::cout << "Not primitive" << std::endl;
}
}
In order to save the function call overhead, use structs:
template<class T>
struct IsPrimitiveType {
enum { VALUE = 0 };
};
template<>
struct IsPrimitiveType<int> {
enum { VALUE = 1 };
};
// ...
template<class T>
void test() {
if (IsPrimitiveType<T>::VALUE) {
// ...
} else {
// ...
}
}
As others have pointed out, you can save your time implementing that by yourself and use is_fundamental from the Boost Type Traits Library, which seems to do exactly the same.
Boost TypeTraits has plenty of stuff.
I guess this can do the job quite nicely, without multiple specializations:
# include <iostream>
# include <type_traits>
template <class T>
inline bool isPrimitiveType(const T& data) {
return std::is_fundamental<T>::value;
}
struct Foo {
int x;
char y;
unsigned long long z;
};
int main() {
Foo data;
std::cout << "isPrimitiveType(Foo): " << std::boolalpha
<< isPrimitiveType(data) << std::endl;
std::cout << "isPrimitiveType(int): " << std::boolalpha
<< isPrimitiveType(data.x) << std::endl;
std::cout << "isPrimitiveType(char): " << std::boolalpha
<< isPrimitiveType(data.y) << std::endl;
std::cout << "isPrimitiveType(unsigned long long): " << std::boolalpha
<< isPrimitiveType(data.z) << std::endl;
}
And the output is:
isPrimitiveType(Foo): false
isPrimitiveType(int): true
isPrimitiveType(char): true
isPrimitiveType(unsigned long long): true
The following example (first posted in comp.lang.c++.moderated) illustrates using partial specialisation to print things differently depending on whether or not they are built-in types.
// some template stuff
//--------------------
#include <iostream>
#include <vector>
#include <list>
using namespace std;
// test for numeric types
//-------------------------
template <typename T> struct IsNum {
enum { Yes = 0, No = 1 };
};
template <> struct IsNum <int> {
enum { Yes = 1, No = 0 };
};
template <> struct IsNum <double> {
enum { Yes = 1, No = 0 };
};
// add more IsNum types as required
// template with specialisation for collections and numeric types
//---------------------------------------------------------------
template <typename T, bool num = false> struct Printer {
void Print( const T & t ) {
typename T::const_iterator it = t.begin();
while( it != t.end() ) {
cout << *it << " ";
++it;
}
cout << endl;
}
};
template <typename T> struct Printer <T, true> {
void Print( const T & t ) {
cout << t << endl;
}
};
// print function instantiates printer depoending on whether or
// not we are trying to print numeric type
//-------------------------------------------------------------
template <class T> void MyPrint( const T & t ) {
Printer <T, IsNum<T>::Yes> p;
p.Print( t );
}
// some test types
//----------------
typedef std::vector <int> Vec;
typedef std::list <int> List;
// test it all
//------------
int main() {
Vec x;
x.push_back( 1 );
x.push_back( 2 );
MyPrint( x ); // prints 1 2
List y;
y.push_back( 3 );
y.push_back( 4 );
MyPrint( y ); // prints 3 4
int z = 42;
MyPrint( z ); // prints 42
return 0;
}
There is a better way - using SFINAE. With SFINAE you don't have to list out every primitive type. SFINAE is a technique that depends on the idea that when template specialization fails it falls back to a more general template. ( it stands for "Specialization failure is not an error" ).
Also you don't really define if you consider a pointer to be a primitive type, so I'll make you templates for all the combinations.
// takes a pointer type and returns the base type for the pointer.
// Non-pointer types evaluate to void.
template < typename T > struct DePtr { typedef void R; };
template < typename T > struct DePtr< T * > { typedef T R; };
template < typename T > struct DePtr< T * const > { typedef T R; };
template < typename T > struct DePtr< T * volatile > { typedef T R; };
template < typename T > struct DePtr< T * const volatile > { typedef T R; };
// ::value == true if T is a pointer type
template < class T > struct IsPointer { enum { value = false }; };
template < class T > struct IsPointer < T * > { enum { value = true }; };
template < class T > struct IsPointer < T * const > { enum { value = true }; };
template < class T > struct IsPointer < T * volatile > { enum { value = true }; };
template < class T > struct IsPointer < T * const volatile > { enum { value = true }; };
// ::value == true if T is a class type. ( class pointer == false )
template < class T > struct IsClass
{
typedef u8 yes; typedef u16 no;
template < class C > static yes isClass( int C::* );
template < typename C > static no isClass( ... );
enum { value = sizeof( isClass<T>( 0 )) == sizeof( yes ) };
};
// ::value == true if T* is a class type. ( class == false )
template < class T > struct IsClassPtr
{
typedef u8 yes; typedef u16 no;
template < class C > static yes isClass( int C::* );
template < typename C > static no isClass( ... );
enum { value = sizeof( isClass< typename DePtr< T >::R >( 0 )) == sizeof( yes ) };
};
// ::value == true if T is a class or any pointer type - including class and non-class pointers.
template < class T > struct IsClassOrPtr : public IsClass<T> { };
template < class T > struct IsClassOrPtr < T * > { enum { value = true }; };
template < class T > struct IsClassOrPtr < T * const > { enum { value = true }; };
template < class T > struct IsClassOrPtr < T * volatile > { enum { value = true }; };
template < class T > struct IsClassOrPtr < T * const volatile > { enum { value = true }; };
template < class T > struct IsClassOrClassPtr : public IsClass<T> { };
template < class T > struct IsClassOrClassPtr < T * > : public IsClassPtr< T* > { };
template < class T > struct IsClassOrClassPtr < T * const > : public IsClassPtr< T* const > { };
template < class T > struct IsClassOrClassPtr < T * volatile > : public IsClassPtr< T* volatile > { };
template < class T > struct IsClassOrClassPtr < T * const volatile > : public IsClassPtr< T* const volatile > { };
It cannot be done exactly the way you asked. Here is how it can be done:
template<class T>
class A{
void doWork(){
bool isPrimitive = boost::is_fundamental<T>::value;
if(isPrimitive)
doSomething();
else
doSomethingElse();
}
private:
T *t;
};
You may get a warning if you put the value of isPrimitive directly inside the if statement. This is why i introduced a temporary variable.
Yet another similar examples:
#include <boost/type_traits/is_fundamental.hpp>
#include <iostream>
template<typename T, bool=true>
struct foo_impl
{
void do_work()
{
std::cout << "0" << std::endl;
}
};
template<typename T>
struct foo_impl<T,false>
{
void do_work()
{
std::cout << "1" << std::endl;
}
};
template<class T>
struct foo
{
void do_work()
{
foo_impl<T, boost::is_fundamental<T>::value>().do_work();
}
};
int main()
{
foo<int> a; a.do_work();
foo<std::string> b; b.do_work();
}
Assuming by 'Primitive Type' you mean the built in types you can do a series of template specializations. Your code would become:
template<class T>
struct A{
void doWork();
private:
T *t;
};
template<> void A<float>::doWork()
{
doSomething();
}
template<> void A<int>::doWork()
{
doSomething();
}
// etc. for whatever types you like
template<class T> void A<T>::doWork()
{
doSomethingElse();
}
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