Question

Can somebody explain to me why the following works:

template<class T> class MyTemplateClass {
public:
    T * ptr;
};

int main(int argc, char** argv) {
    MyTemplateClass<double[5]> a;
    a.ptr = new double[10][5];
    a.ptr[2][3] = 7;
    printf("%g\n", a.ptr[2][3]);
    return 0;
}

But this doesn’t:

class MyClass {
public:
    double[5] * ptr;
    // double(*ptr)[5]; // This would work
};

int main(int argc, char** argv) {
    MyClass a;
    a.ptr = new double[10][5];
    a.ptr[2][3] = 7;
    printf("%g\n", a.ptr[2][3]);
    return 0;
}

Obviously there is more to template instantiation than just a textual replacement by the arguments to the template – is there a simple explanation of this magic?

For the latter the compiler (g++ 4.1.2) spits out the following error:

test.cxx:13: error: expected unqualified-id before '[' token

Where line 13 is the double[5] * ptr; line.

The question is not:

“Why does the MyClass example fail? – because C++ doesn’t allow Java style array declarations ;-)”.

But is:

“Why does the MyTemplateClass example succeed?”

Answer 1

The difference lies in the C++ grammar. A simple-declaration is formed like this:

declaration-specifier-seq init-declarator-list

Where declaration-specifier-seq is a sequence of declaration specifiers:

simple-type-specifier: int, bool, unsigned, typedef-name, class-name ...
class-specifiers: class X { ... }
type-qualifier: const, volatile
function-specifier: inline, virtual, ... 
storage-class-specifier: extern, static, ...
typedef

You get the idea. And init-declarator-list is a list of declarators with an optional initializer for each:

a
*a
a[N]
a()
&a = someObj

So a full simple-declaration could look like this, containing 3 declarators:

int a, &b = a, c[3] = { 1, 2, 3 };

Class members have special rules to account for the different context in which they appear, but they are very similar. Now, you can do

typedef int A[3];
A *a;

Since the first uses the typedef specifier and then simple-type-specifier and then a declarator like “a[N]”. The second declaration then uses the typedef-name “A” (simple-type-specifier) and then a declarator like “*a”. However, you of course cannot do

int[3] * a;

Since “int[3]” is not a valid declaration-specifier-seq as shown above.

And now, of course, a template is not just like a macro text substitution. A template type parameter of course is treated like any other type-name which is interpreted as just the type it names and can appear where a simple-type-specifier can appear. Some C# folks tend to say C++ templates are “just like macros”, but of course they are not 🙂