CODEWITHSUNDEEP
c++templatesdefault-parameters
Cameron
Cameron

Reputation: 98746

Is it valid to have default template arguments that are dependent on earlier arguments?

For example, the following snippet compiles in VC++ 2010:

template<int Rows, int Columns = Rows>
struct Matrix {  };

Matrix<4> m;

Note that the default argument for Columns depends on the argument value for Rows.

But is this standard behaviour in C++11 (or earlier) that I can rely on everywhere?

Upvotes: 5

Views: 763

Answers (2)

phoeagon
phoeagon

Reputation: 2090

Yes. And as a matter of fact, it's how tons of STL codes work.

the std::vector has a definition like:

 template < class T, class Alloc = allocator<T> > class vector

so that you don't need to specify the allocator each time every time. If such is invalid, we won't be able to write:

std::vector<int> data;

And you would write std::map as:

std::map < keyType,                                     // map::key_type
       ValType,                                       // map::mapped_type
       less<keyType>,                     // map::key_compare
       allocator<pair<const KeyType,ValType> >    // map::allocator_type
       > mapping;

which is far less desirable than:

std::map< keyType , ValType > mapping;

Upvotes: 4

Leo supports Monica Cellio
Leo supports Monica Cellio

Reputation: 6037

According to cplusplus, yes:

It is also possible to set default values or types for class template parameters. For example, if the previous class template definition had been:

template <class T=char, int N=10> class mysequence {..};

And on a more mundane note, g++ -Wall will compile it.

Upvotes: 2

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