declaration order of member variables in struct

[Michael Eager]

Martin Ettl wrote:
> Hello,
> 
> i have following source, that compiles when the order of the declarations is as following:
> 
> #include <iostream>
> #include <cstring>
> 
> template <int N, int Low=1, int Upp=N> struct Root 
> {
>   static const int  mean = (Low+Upp)/2;
>   static const bool down = ((mean*mean)>=N);
>   static const int  ret  = Root<N,(down?Low:mean+1),(down?mean:Upp)>::ret;
> }; 
> template <int N, int Mid> struct Root<N,Mid,Mid>
> {static const int ret = Mid;};
> 
> int main()
> {
> 	std::cout << Root<10,1,10>::ret << std::endl;
> }
> 
> But, when i reorder the declaration sequence in struct Root as followed:
> 
> template <int N, int Low=1, int Upp=N> struct Root 
> {
>   static const int  ret  = Root<N,(down?Low:mean+1),(down?mean:Upp)>::ret;
                                      ^^^^     ^^^^     ^^^^ ^^^^

Neither down nor mean are defined before they are referenced.

>   static const int  mean = (Low+Upp)/2;
>   static const bool down = ((mean*mean)>=N);
> };
> 
> g++-4.4.1 complains about and prints the following output:
> 
> ../src/TestRoot.cpp:8: error: ‘down’ was not declared in this scope
> ../src/TestRoot.cpp:8: error: ‘mean’ was not declared in this scope
> ../src/TestRoot.cpp:8: error: ‘down’ was not declared in this scope
> ../src/TestRoot.cpp:8: error: ‘mean’ was not declared in this scope

Two references to 'down' and two references to 'mean', not declared
before being used.

> ../src/TestRoot.cpp:8: error: template argument 2 is invalid
> ../src/TestRoot.cpp:8: error: template argument 3 is invalid

Arguments 2 and 3 contain undefined variables.

> 
> I do not understand why this depends on the order of declaration?

Each symbol (variable or type) in C/C++ must be declared before it is used.


-- 
Michael Eager	 eager@eagercon.com
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