On Jun 26, 8:08 am, John Harrison <john_androni...@hotmail.comwrote:
subramanian10...@yahoo.com, India wrote:
Consider the program
#include <iostream>
using namespace std;
class Test
{
public:
Test(Test_int c_value)
{
value = c_value;
}
typedef int Test_int;
private:
Test_int value;
};
int main()
{
Test t(100);
return 0;
}
Here I am getting compilation error for the line
Test(Test_int c_value)
because it uses Test_int type which is not known at that point and is
declared afterwards.
If I declare
typedef int Test_int;
before the the ctor, the compilation error goes. But the data member
'value' is used inside the ctor at which point the definition of the
variable is not known(it is defined later in the private section).
My doubt is why the compiler is unable to see the typedef declaration
for Type_int when it occurs later in the class; but it is able to see
the data member 'value' used in the ctor but which is actually defined
inside the private section later in the class.
Why are the typedef and variable definition inside the class treated
differently ?
My understanding is that this rule exists to make life easier for
compiler writers. The C/C++ syntax is ambiguous (in the LALR(1) sense)
unless you know which names are typedefs and which names are just
ordinary identifiers. So special requirements are made of typedefs,
namely that you must declare them before you use them.
The rule you're thinking of applies to templates, and requires
the use of typename in some cases. Here, the rule has nothing
to do with whether the symbol is a type. Within function
bodies, default arguments, exception-specifications, and
constructor ctor-initializers (including such things in nested
classes), the class is considered complete; it's as if these
things were parsed after the closing brace of the class
definition. So using Test_int or value within the function
definition is legal, but using it before the opening brace of
the function (except as a default argument or in an exception
specifier) isn't.
--
James Kanze (GABI Software) email:ja*********@gmail.com
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