Menu
I have a c++ header file containing a class. I want to use this class in several projects, bu I don't want to create a separate library for it, so I'm putting both methods declarations and definitions in the header file:
// example.h #ifndef EXAMPLE_H_ #define EXAMPLE_H_ namespace test_ns{ class TestClass{ public: void testMethod(); }; void TestClass::testMethod(){ // some code here... } } // end namespace test_ns #endif If inside the same project I include this header from more than one cpp file, I get an error saying "multiple definition of test_ns::TestClass::testMethod()", while if I put the method definition inside the class body this does not happen:
// example.h #ifndef EXAMPLE_H_ #define EXAMPLE_H_ namespace test_ns{ class TestClass{ public: void testMethod(){ // some code here... } }; } // end namespace test_ns #endif Since the class is defined inside a namespace, shouldn't the two forms be equivalent? Why is the method considered to be defined twice in the first case?
976
To avoid multiple inclusions of the same header file we use the #ifndef, #define and #endif preprocessor directives. Just write the entire program in only file and include headers once only. You can use the conditional preprocessor directive named #ifndef to check whether that symbolic name has already been assigned.
If a header file happens to be included twice, the compiler will process its contents twice. This is very likely to cause an error, e.g. when the compiler sees the same structure definition twice. Even if it does not, it will certainly waste time. This construct is commonly known as a wrapper #ifndef.
The output of the above program. Including Multiple Header Files: You can use various header files in a program.
Yes, you can use #include<bits/stdc++. h> This header file includes all the standard header files. The only problem is, it would include a lot of unnecessary files which you don't require for a program, and also, since it includes a lot of header files your compilation time will increase.
These are not equivalent. The second example given has an implicit 'inline' modifier on the method and so the compiler will reconcile multiple definitions itself (most likely with internal linkage of the method if it isn't inlineable).
The first example isn't inline and so if this header is included in multiple translation units then you will have multiple definitions and linker errors.
Also, headers should really always be guarded to prevent multiple definition errors in the same translation unit. That should convert your header to:
#ifndef EXAMPLE_H #define EXAMPLE_H //define your class here #endif If you love us? You can donate to us via Paypal or buy me a coffee so we can maintain and grow! Thank you!
Donate Us With
