C++ for Scientists - Technische Universität Dresden

140 CHAPTER 5. META-PROGRAMMING
First of all, is the ref member really constant? We never used const in the class definition or
the function trans. Help is provided from the ‘Run-Time Type Identification (RTTI)’. We add
the header ‘typeinfo’ and print the type information:
#include
...
std::cout ≪ ”typeid of trans(A) = ” ≪ typeid(tst::trans(A)).name() ≪ ’\n’;
std::cout ≪ ”typeid of trans(B) = ” ≪ typeid(tst::trans(B)).name() ≪ ’\n’;
This will produce the following output: 4
typeid of trans(A) = N3tst15transposed_viewIN3mtl6matrix7dense2DIfNS2_10
parametersINS1_3tag9row_majorENS1_5index7c_indexENS1_9non_fixed10
dimensionsELb0EEEEEEE
typeid of trans(B) = N3tst15transposed_viewIKN3mtl6matrix7dense2DIfNS2_10
parametersINS1_3tag9row_majorENS1_5index7c_indexENS1_9non_fixed10
dimensionsELb0EEEEEEE
The output is apparently not very clear. However, if we look very careful, we see the extra
‘K’ in the second line that tells us that the view is instantiated with a constant matrix type.
Another disadvantage of RTTI is that we only see the const attribute of template parameters.
That is printing the type informantion of trans(B).ref would not tell wether or not this type is
constant.
An alternative that solves both problems is inspecting the type by provocing an error message.
We can for instance write:
int ta= trans(A);
int tb= trans(B);
Then the compiler gives us a message like:
trans_const.cpp:120: Error: ≫mtl::matrix::transposed_view >≪ cannot be converted to ≫int≪
in initialization
trans_const.cpp:121: Error: ≫const mtl::matrix::transposed_view≪ cannot be
converted to ≫int≪ in initialization
Here the types are much more readable. 5 We can see clearly that trans(B) returns a view with
a constant template parameter. The same trick could be done for the reference in the view:
int tar= trans(A).ref;
int tbr= trans(B).ref;
The error message would be accordingly:
4
With g++, on other compilers it might be different but the essential information will be the same. The lines
are broken manually.
5
TODO: Why the hell is this const outside in line 121???