Question

I started getting the error, “error C2059: syntax error : ‘default argument'” for a line of code that declared a function with a string argument that was given a default parameter. This was obviously a bit frustrating, as the error message was not exactly enlightening (I know it’s a ‘default argument’!), and the exact declaration would work elsewhere.

After shifting about the declaration a bit, I found its position in its containing class actually had an effect. Narrowing it down, I found that I was declaring a different function somewhat erroneously, by including a semicolon after one of its default parameters. The compiler seemed perfectly fine with that, which seemed a bit odd. I investigated a bit more, and came up with the following test case to try to figure out the essence of what was going on:

enum TestEnum1
{
   TEST_ONE
};
class TestClass
{
public:
   enum TestEnum2
   {
      TEST_TWO,
      TEST_THREE,
      TEST_FOUR
   };
   void Func1( int iParm = TEST_ONE; ); // additional semicolon here
   void Func2( std::string strParm = "" );
};

As the code above stands, Func2 will produce the compilation error I mentioned above. If I move Func2 above Func1, then everything compiles fine.

If I switch the default parameter in Func1 to an explicit number or use an enum declared within TestClass, then I get an expected syntax error for that line.

So essentially, the strange thing seems to be that if I set a default parameter’s value to an enum not defined directly in the current class and am a little too semicolon-happy, the compiler will ignore the syntax error, until some other seemingly-unrelated thing finally causes the parser to die in a very inscrutable way.

Am I just missing something completely? Is this expected behavior? I’m hesitant to go calling it a bug in the compiler, certainly, but this hardly seems correct. If it’s just me misunderstanding something about the standard, then I’d like to know where I’m wrong.

Answer 1

Agreed with @tlayton. Having dabbled a bit in parsers myself, I can attest that generating good error messages for syntax errors that confuse the parser’s sense of scope can be very hard to do.

This particular case is however close to a defect. The irony is that in VS2010, the compiler still generates the same lousy error message but the IntelliSense parser actually catches it:

3   IntelliSense: expected a ')'    c:\projects\cpptemp14\cpptemp14.cpp 20  36  cpptemp14

That’s borked. You can report it at connect.microsoft.com. Let me know if you don’t want to take the time, I’ll report it (MVP duty).