Writing function definition in header files in C++

by

If the function is small (the chance you would change it often is low), and if the function can be put into the header without including myriads of other headers (because your function depends on them), it is perfectly valid to do so. If you declare them extern inline, then the compiler is required to give it the same address for every compilation unit:

headera.h:

inline string method() {
    return something;
}

Member functions are implicit inline provided they are defined inside their class. The same stuff is true for them true: If they can be put into the header without hassle, you can indeed do so.

Because the code of the function is put into the header and visible, the compiler is able to inline calls to them, that is, putting code of the function directly at the call site (not so much because you put inline before it, but more because the compiler decides that way, though. Putting inline only is a hint to the compiler regarding that). That can result in a performance improvement, because the compiler now sees where arguments match variables local to the function, and where argument doesn’t alias each other – and last but not least, function frame allocation isn’t needed anymore.

My understanding is when the compilation is done, compiler will expand the header file and place it where it is included. Is that correct?

Yes, that is correct. The function will be defined in every place where you include its header. The compiler will care about putting only one instance of it into the resulting program, by eliminating the others.

More Related Contents:

  1. Regarding time out in code
  2. What is a “cache-friendly” code?
  3. Why aren’t my include guards preventing recursive inclusion and multiple symbol definitions?
  4. Why should I not include cpp files and instead use a header?
  5. Modular arithmetics and NTT (finite field DFT) optimizations
  6. C++ performance challenge: integer to std::string conversion
  7. Replacing a 32-bit loop counter with 64-bit introduces crazy performance deviations with _mm_popcnt_u64 on Intel CPUs
  8. Header guards in C++ and C
  9. Ternary operator ?: vs if…else
  10. Difference between iostream and iostream.h
  11. Is inline assembly language slower than native C++ code?
  12. Trial-division code runs 2x faster as 32-bit on Windows than 64-bit on Linux
  13. vector or map, which one to use?
  14. Performance issue for vector::size() in a loop in C++
  15. 5 years later, is there something better than the “Fastest Possible C++ Delegates”?
  16. Generating m distinct random numbers in the range [0..n-1]
  17. Is the ranged based for loop beneficial to performance?
  18. What is the performance cost of having a virtual method in a C++ class?
  19. Why is iterating though `std::vector` faster than iterating though `std::array`?
  20. Difference between rdtscp, rdtsc : memory and cpuid / rdtsc?
  21. c++11 regex slower than python
  22. Self-sufficient header files in C/C++
  23. Why is processing an unsorted array the same speed as processing a sorted array with modern x86-64 clang?
  24. Which is faster: x
  25. Benefits of using reserve() in a vector – C++
  26. Does auto deduce the type at compile time or runtime in C++ 11?
  27. Should I include stddef.h or cstddef for size_t
  28. C++ Serialization Performance
  29. Measuring exception handling overhead in C++
  30. Why is pow(int, int) so slow?

Leave a Comment