What's the point of g++ -Wreorder?

c++ g++ compiler-warnings
Peeter Joot picture Peeter Joot · Dec 1, 2009 · Viewed 51.4k times · Source

The g++ -Wall option includes -Wreorder. What this option does is described below. It is not obvious to me why somebody would care (especially enough to turn this on by default in -Wall).

-Wreorder (C++ only)
  Warn when the order of member initializers given in the code does not
  match the order in which they must be executed.  For instance:

    struct A {
      int i;
      int j;
      A(): j (0), i (1) { }
    };

  The compiler will rearrange the member initializers for i and j to
  match the declaration order of the members, emit-ting a warning to that
  effect.  This warning is enabled by -Wall.

Answer

int3 picture int3 · Dec 1, 2009

Consider:

struct A {
    int i;
    int j;
    A() : j(0), i(j) { }
};

Now i is initialized to some unknown value, not zero.

Alternatively, the initialization of i may have some side effects for which the order is important. E.g.

A(int n) : j(n++), i(n++) { }

Related questions

Why should I initialize member variables in the order they're declared in?

I was writing some code today and got a weird compile error, which seems to be caused by initializing member variables in a different order than they were declared. Example: class Test { int a; int b; public: Test() : b(1), a(2) { } }; …

Read More
g++ How to get warning on ignoring function return value

lint produces some warning like: foo.c XXX Warning 534: Ignoring return value of function bar() From the lint manual 534 Ignoring return value of function 'Symbol' (compare with Location) A function that returns a value is called just for side effects …

Read More
Understanding -Weffc++

Consider the following program: #include <string> struct S { S (){} private: void *ptr = nullptr; std::string str = ""; }; int main(){} This, when compiled with -Weffc++ on GCC 4.7.1, will spit out: warning: 'struct S' has pointer data members [-Weffc++] warning: but …

Read More