pure/const functions in C++0x

Question

In C++98/C++03, there are no pure/const function keywords in the language.

1. Has this changed in C++0x?

2. If so, is it possible to set such a flag even on function objects (std::function)? So I can pass some function pointer or lambda functions and additional give the information that it is a pure/const function? The called function may have an optimized execution path for such a function.

3. Is there some way to check if a given function is pure/const? I.e. for example, if there is such flag on std::function as described above, I probably could just check that flag. But maybe there is even a more general way.

If it has not changed, why not? I think it might be quite useful to have such a support.

Are there any open proposals about it?

Answer 1

1. Has this changed in C++0x?

No. There is a constexpr but it means compile time constant. If its parameters are constexprs too then it's executed at compile time, but it's a regular function otherwise. Since they must be defined in the same translation unit and consist of a single return statement they probably will be inlined and the above optimization will be performed. It can't be used to provide the compiler information about externally linked function.

If it has not changed, why not? I think it might be quite useful to have such a support.

Actually I don't think you need it. The language is already too big, and the programmer can easily rewrite this code to be more efficient based on her knowledge anyway. Unlike restrict it doesn't provide any info that can't be expressed by other means.

Are there any open proposals about it?

I haven't seen any committee papers on that topic.

Answer 2

gcc uses __attribute__(( <attr> )) to define extra attributes on functions.

• pure: only accesses (but does not modify) parameters and global memory. GCC uses this information to determine if the optimizer can completely omit repeated calls to the function (local-memoization). Two notable pure functions are strlen and memcmp.

• const: not to be confused with C++ const, const functions only accesses parameters and those parameters must not be pointers. It is basically a more restricted version of pure. The optimizer treats const functions that same as pure. Though in theory it could perform more aggressive (non-local) memoization than it does for pure.

C++11's new attribute syntax (§7.6) was designed to do just this sort of thing. Currently you cannot use C++'s attribute syntax to set GCC attributes, but that will change in future gcc versions.

So you will be able to assign the pure attribute to functions using the attribute syntax. But there isn't a standard pure attribute. pure will be compiler specific, but it will do the right thing on gcc.

For the curious, here is the list of the standard attributes:

• align
• noreturn
• override
• hiding
• base_check
• carries_dependency