为什么这个 const 说明符有未指定的行为？

Question

I maintain an open source program and one of my users reported that it won't compile under clang , which I've never used before. One of the errors that I'm getting is *Warning: qualifier on function type 'junky_t' (aka 'void (const int &, const int &)') has unspecified behavior.* . I've created a small program that demonstrates the issue:

typedef void junky_t(const int &foo,const int &bar);

class demo {
public:;
    const junky_t *junk;
};

And here's what happens when I try to compile:

$clang -DHAVE_CONFIG_H  -g -g -O3 -Wall -MD -Wpointer-arith -Wshadow -Wwrite-strings -Wcast-align -Wredundant-decls -Wdisabled-optimization -Wfloat-equal -Wmultichar -Wmissing-noreturn -Woverloaded-virtual -Wsign-promo -funit-at-a-time -Weffc++  -D_FORTIFY_SOURCE=2  -MT demo.o -MD -MP -c -o demo.o demo.cpp

demo.cpp:5:5: warning: qualifier on function type 'junky_t' (aka 'void (const int &, const int &)') has unspecified behavior
    const junky_t *junk;
    ^~~~~~~~~~~~~
1 warning generated.

That is, class demo has a function pointer to a function that has a signature that takes a number of const references. The const in the class demo is supposed to prevent junk from being changed. However apparently it's ambiguous because the function itself might be considered const , which it is not. I do not have a problem compiling this with gcc or llvm but it will not compile with clang on a Mac. What should I do?

Answer 1

This is not unspecified behavior . The warning of clang is wrong. Your code is legal c++. The Standard (C++11) states, that cv-qualifiers on top of a function type are ignored.

Hence, it doesn't make any sense to put a const qualifier on top of a function type. If you want your pointer to be const then write

junky_t * const junk;

otherwise just write

junky_t * junk;