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Is it permissible to have default template arguments that depend on previous arguments?

For example, the following snippet compiler 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 the standard behavior in C ++ 11 (or earlier) that I can rely on everywhere?

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Yes. And, essentially, this is how tons of STL codes work.

std::vector has a definition like:

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

so you don’t have to specify allocator every time every time. If this is not valid, we will not be able to write:

 std::vector<int> data;

And you should 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 much less desirable than:

 std::map< keyType , ValType > mapping;
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According to cplusplus , yes:

You can also set default values ​​or types for class template parameters. For example, if the previous class template definition was:

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

And in a more mundane note, g ++ -Wall compiles it.

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