Are functions in C/C++ headers a no-go?_问答_开发者

I'm working on a very tiny piece of C/C++ source code. The program reads input values from stdin, processes them with an algorithm and writes the results to stdout.

I would just implement all that in a single file, but 开发者_StackOverflow社区I also want test cases for the algorithm (not the input/output reading), so I have the following files in my project:

main.cpp
sort.hpp
sort_test.cpp

I implement the algorithm in sort.hpp right away, no sort.cpp. It's rather short and doesn't have any dependencies.

Would you say that, in some cases, functions defined in headers are okay, even if they are sophisticated algorithms and not just simple accessors/mutators? Or is there a reason I should avoid this? When should I move code from header to source file?

There is nothing wrong with having functions in header files, as long as you understand the tradeoff. Putting them in a header file means they'll have to be compiled (and recompiled) in any translation unit that includes the header. (and they have to be declared inline, or you will get linker errors.)

In projects with many translation units, that may add up to a noticeable slowdown in compile times, if you do it a lot.

On the other hand, it ensures that the function definition is visible everywhere the function is called -- and that means that it can be trivially inlined, so the resulting program may run faster.

And finally, with function templates, you typically have no realistic alternative. The definition must be visible at the call site, and the only practical way to achieve that is to put it in a header.

A final consideration is that header-only libraries are easier to deploy and use. You don't need to link against anything, you don't have to worry about ABI's or anything else. You just add the headers to your project, include them and off you go.

Quite a few popular libraries use a header-only strategy.

When you put functions in headers you have to make sure to declare them inline. This is required to avoid a duplicate definition warning when more than one .cpp file include that header file. Generally you should only put small functions inside header files because it will be compiled for each cpp file that includes the header which will slow down compilation time and also results in code bloat; a larger executable file.

It's OK to put any function in the header as long as it's inline. Things such as functions defined inside class { } and templates are implicitly inline.

If the resulting application becomes too large, then optimize the code size. Optimizing before there is a problem is an anti-pattern, especially when there is a benefit to doing it "your way," and the fix is as simple as moving from one file to another and erasing inline.

Of course, if you want to distribute the code as a library, then deciding between a header, static library, or dynamic library binary is an important decision affecting the users.

The vast majority of the boost libraries are header-only, so I'd say: Yes, this is an established and accepted practice. Just don't forget to inline.

That really is a stile choice. But putting it in the header does mean that it will be inline code rather than a function. If you wanted that same functionality, you could use the inline keyword:

inline int max(int a, int b)
{
  return (a > b) ? a : b;
}

http://en.wikipedia.org/wiki/Inline_function

The reason you should avoid this in general (for non inline functions) is because multiple source files will be including your header, creating linker errors. It doesn't matter if you have a pramga once or similar trick - the duplication will show up if you have more than one compilation unit (e.g. cpp files) including the same header.

If you wish to inline the function, it MUST be in the header else it can't get inlined.

If you publish a header with your libraries and the header has some sort of implementation in it, you can be sure that after a few years if you change the implementation and it doesn't work exactly the same way as it did before, some peoples code will break since thay will have come to rely on the implementation they saw in the header. Yeah i know one should not do it but many people do look in header for the implementation and other behaviour they can exploit/use in a not intended way to overcome some problem they are having.

If you are planning to use templates then you have no choice but to put it all in header. (this might not be necessary if you compiler supports export templates but there is only 1 i know of).

Its ok to have the implementation in the header. It depends on what you need. If you separate the definition to a different file then the compiler will create symbols with external linkage if you dont want that you can define the functions inside the header itself. But you would be wasting some amount of memory for the code segment. If you include this header file in two different files then both files codes segment will have this function definition.

If other header file is going to have a function with similar name then its going to be a problem. Then you have to use inline.