__if_exists Statement
The __if_exists statement tests whether the specified identifier exists. If the identifier exists, the specified statement block is executed.
__if_exists ( identifier ) {
statements
};
Parameters
Parameter |
Description |
|---|---|
identifier |
The identifier whose existence you want to test. |
statements |
One or more statements to execute if identifier exists. |
Remarks
Warning
To achieve the most reliable results, use the __if_exists statement under the following constraints.
Apply the __if_exists statement to only simple types, not templates.
Apply the __if_exists statement to identifiers both inside or outside a class. Do not apply the __if_exists statement to local variables.
Use the __if_exists statement only in the body of a function. Outside of the body of a function, the __if_exists statement can test only fully defined types.
When you test for overloaded functions, you cannot test for a specific form of the overload.
The complement to the __if_exists statement is the __if_not_exists statement.
Example
Notice that this example uses templates, which is not advised.
// the__if_exists_statement.cpp
// compile with: /EHsc
#include <iostream>
template<typename T>
class X : public T {
public:
void Dump() {
std::cout << "In X<T>::Dump()" << std::endl;
__if_exists(T::Dump) {
T::Dump();
}
__if_not_exists(T::Dump) {
std::cout << "T::Dump does not exist" << std::endl;
}
}
};
class A {
public:
void Dump() {
std::cout << "In A::Dump()" << std::endl;
}
};
class B {};
bool g_bFlag = true;
class C {
public:
void f(int);
void f(double);
};
int main() {
X<A> x1;
X<B> x2;
x1.Dump();
x2.Dump();
__if_exists(::g_bFlag) {
std::cout << "g_bFlag = " << g_bFlag << std::endl;
}
__if_exists(C::f) {
std::cout << "C::f exists" << std::endl;
}
return 0;
}
Output
In X<T>::Dump()
In A::Dump()
In X<T>::Dump()
T::Dump does not exist
g_bFlag = 1
C::f exists