C++ preprocessor/macro to automatically add lines after function definition

Question

In C++ I want to make functions that when declared, gets automatically added to a map( or vector, doesn't really matter in this case) as a function pointer and is called later automatically. For example this would be useful if I am writing unit test framework and I just want users to declare each of their unit tests like this:

UNIT_TEST_FUNCTION(function_name){

// do something 

}

and instead something like this gets called

void function_name(){
  //do something
}
int temp = register_function("function_name", function_name);

Where register_function() adds the user defined function in a map of function pointers for example. So basically, I need a mechanism that adds additional lines of code after a function definition, so that some action is performed automatically on the defined function. Is this possible using macros perhaps?

A macro can only generate a consecutive block of text, so no it can't do what you want. — Mark Ransom
– Mark Ransom, Commented Feb 28, 2013 at 18:08

Mark Ransom · Accepted Answer · 2013-02-28 18:13:43Z

7

A macro can only generate a consecutive block of text. It can't lay things out the way you show in the question.

However if you're willing to rearrange a little, it can be done.

#define UNIT_TEST_FUNCTION(function_name) \
    void function_name(); // forward declaration \
    int temp##function_name = register_function(#function_name, function_name); \
    void function_name()

answered Feb 28, 2013 at 18:13

Mark Ransom

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1 Comment

Matthew Sanders Over a year ago

I voted this up because it satisfies the question as is. I would also like to point out that it is much better to first attempt an object based approach, fallback on to a template metaprogramming approach, and if that doesn't work as a last resort use a macro. Macros are harder to maintain, debug, and do no sanity checking. I have found them useful in automating serialization of objects due to the string concatenation (##).

John Dibling · Accepted Answer · 2013-02-28 18:14:53Z

1

A single preprocessor macro can't do what you want because it can only generate a single, contiguous block of text. Preprocessor macros are stupid in the sense that they don't understand anything about the language -- hence the preprocessor in 'preprocessor macro'.

What you can do is use a pair of macros or tuple of macros to delimit the begin and end of your test case mapping, and a single macro for each individual test case. Something along these lines:

TEST_CASES_BEGIN

UNIT_TEST_FUNCTION(function_name){

// do something 

}

TEST_CASES_END

The Boost unit test facility uses a mechanism very similar to this. You might even (eventually) find this design to be a little more expressive than the design you are trying to achieve.

answered Feb 28, 2013 at 18:14

John Dibling

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1 Comment

Baruch Over a year ago

Can UNIT_TEST_FUNCTION be used to build up TEST_CASES_END, so that it will have a line for each call to UNIT_TEST_FUNCTION?

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