Question

Alternate question title would be:
How to explicitly have the compiler generate code for the compiler-generated constructors in a specific translation unit?

The problem we face is that for one code path the resulting — thoroughly measured — performance is better (by about 5%) if the copy-ctor calls of one object are not inlined, that is if this constructor is implemented manually. (We noticed this because during code-cleanup the superfluous explicitly implemented copy ctor of this class (17 members) was removed.)

Edit: Note that we have checked the generated assembly code and have made sure that the inlining and code generation is happening as I describe for the two different code versions.

We face now the choice of just dropping the manual copy-ctor code back in (it does exactly the same as the compiler generated one) or finding any other means of not inlining the copy ctor of this class.

Is there any means (for Microsoft Visual C++) to explicitly instantiate the compiler generated class functions in a specific translation unit or will they always be inlined in each translation unit where they are used? (Comments for gcc or other compilers are also welcome to get a better picture of the situation.)

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Since the first 2 answers show some misunderstanding: The compiler generated class functions are only generated by the compiler itself if they are neither declared nor defined by the user. Therefore no modifiers whatsoever can be applied to them, since these function do not exist in the sourcecode.

struct A {
  std::string member;
};

A has a default and copy ctor, a dtor and a copy operator. Neither of these function can be modified via some declspec because they do not exist in the code.

struct B {
  std::string member;
  B(B const& rhs);
};

B now has a user supplied copy ctor and the user has to implement it. The compiler will not generate code for it.

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Some more background for the doubters 🙂 …

This code is compiled using MS Visual C++, but it is linked for an embedded(-like) (realtime) system. Performance was measured by taking timings on this system and I therefore think the guys who took the timings will have some decent numbers.

The test was performed by comparing two code versions where the only difference was the inline vs. the not-inline copy ctor of this one class. Timings with the inlined code were worse by about 5%.

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Further checking has revealed that I was mistaken in one point: The compiler will generate separate functions for complex copy constructors. It will do this on its own discretion and it also depends on the optimization settings. So in our case the compiler is doing the wrong thing in our specific circumstances. From the answers so far it does not appear we can tell the compiler otherwise. 🙁

Answer 1

To add my own conclusion and to answer the exact question without going into details:

1. You cannot force the compiler, specifically VC++, to inline or not inline a compiler-generated ctor/dtor/etc. — but

2. The optimizer will choose – on its discretion – if it inlines the code for a compiler generated function (ctor) or if it generates a “real” function for this code. AFAIK there is no way to influence the decision of the optimizer in this regard.