Default values for array arguments

Question

Just playing around a little with C++. What I really want to do is to be able to setup a function with default values defined for an array or pointer argument. To keep things simple, let's just use an array. Like so:

void experimentA(char a[3] = {'a', 'b', 'c'});

The compiler (LLVM GCC 4.2 with GNU99) complains "Expected expression". That is quite obtuse, but I was told by colleagues that this is happening because the 'value' I'm trying to assign is statically allocated, whereas the variable I'm trying to assign it to (a[3]) is auto.

But I'm not completely sure if that's the case, since I'm able to do this:

void experimentB(char a[3] = "abc");

And the compiler merely warns me that string-literal to char* conversion is deprecated.

I don't understand how "abc" differs fundamentally from {'a','b','c'} in order to cause this discrepancy. Any insight is much appreciated!

Answer 1

Your colleagues are wrong or maybe you misunderstood.

The first clue to understanding is that you cannot have an array as a function parameter in C or C++. The reasons are historical. So when you write void experimentA(char a[3] ...) the compiler automatically converts it to a pointer, i.e. void experimentA(char* a ...). So the real question is why "abc" is a suitable default value for a and { 'a', 'b', 'c' } is not. The reason is as the compiler explains, "abc" is an expression and { 'a', 'b', 'c' } is not (its an initialiser). There are some places in C++ where you can use an initialiser and some where you can't. Default value for a parameter just happens to be one of the places you can't.

Answer 2

When you use string literal "abc", it's allocated by compiler somewhere in the memory, and a pointer to its first character is used as default value. So for the compiler the code is like that: void experimentA(char a[3] = 0x12345678);.

For the second case, the array literal is not allocated by a compiler as a string (which I would see as some inconsistency in the language).

Answer 3

"abc" is an expression. It happens to behave specially, different from all other expressions, when it is used to initialize a variable of type array of char.

{'a','b','c'} is not an expression but an initializer. It is only syntactically allowed in variable definitions. There, the syntax allows either an expression or a non-expression initializer, but that does not man that initializers can be used as expressions anywhere else.