
Question:
Is it possible to define the default value for variables of a template function in C++?
Something like below:
template<class T> T sum(T a, T b, T c=????) { return a + b + c; }
Solution:1
Try this:
template<class T> T sum(T a, T b, T c=T()) { return a + b + c; }
You can also put in T(5) if you are expecting an integral type and want the default value to be 5.
Solution:2
It all depends on the assumptions that you can do about the type.
template <typename T> T sum( T a, T b, T c = T() ) { return a+b+c; } template <typename T> T sum2( T a, T b, T c = T(5) ) { return a+b+c; }
The first case, it only assumes that T
is default constructible. For POD types that is value inititalization (IIRC) and is basically 0
, so sum( 5, 7 )
will call sum( 5, 7, 0 )
.
In the second case you require that the type can be constructed from an integer. For integral types, sum( 5, 7 )
will call sum( 5, 7, int(5) )
which is equivalent to sum( 5, 7, 5 )
.
Solution:3
Yes you can define a default value.
template <class T> T constructThird() { return T(1); } template <class T> T test(T a, T b, T c = constructThird<T>()) { return a + b + c; }
Unfortunately constructThird cannot take a and b as arguments.
Solution:4
Yes, there just needs to be a constructor for T
from whatever value you put there. Given the code you show, I assume you'd probably want that argument to be 0
. If you want more than one argument to the constructor, you could put T(arg1, arg2, arg3)
as the default value.
Solution:5
Yes!
However you should at least have an idea about what T could be or it's useless.
You can't set the default value of template parameters for functions, i.e. this is forbidden:
template<typename T=int> void f(T a, T b);
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