| description | title | ms.date | f1_keywords | helpviewer_keywords | ms.assetid | ||||
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Learn more about: __if_exists Statement | __if_exists Statement | 11/04/2016 |
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| d3eb34b6-f3a9-4063-a286-b62a28c0c7fa |
The __if_exists statement tests whether the specified identifier exists. If the identifier exists, the specified statement block is executed.
__if_exists ( identifier ) { statements }; identifier
The identifier whose existence you want to test.
statements
One or more statements to execute if identifier exists.
Caution
To achieve the most reliable results, use the __if_exists statement under the following constraints.
Apply the
__if_existsstatement to only simple types, not templates.Apply the
__if_existsstatement to identifiers both inside or outside a class. Do not apply the__if_existsstatement to local variables.Use the
__if_existsstatement only in the body of a function. Outside of the body of a function, the__if_existsstatement 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.
Notice that this example uses templates, which is not advised.
// the__if_exists_statement.cpp// compile with: /EHsc #include<iostream>template<typename T> classX : publicT { public:voidDump() { 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; } } }; classA { public:voidDump() { std::cout << "In A::Dump()" << std::endl; } }; classB {}; bool g_bFlag = true; classC { public:voidf(int); voidf(double); }; intmain() { 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; } return0; }In X<T>::Dump() In A::Dump() In X<T>::Dump() T::Dump does not exist g_bFlag = 1 C::f exists 