CODEWITHSUNDEEP
c++stl-algorithm
Skipher
Skipher

Reputation: 215

begin() and end() for STL algorithms

I have a class whose data container is backed by an array, and I have the following implementations for begin() and end().

template <size_t size>
double * MyContainerClass<size>::begin(){
    return std::begin(mContainer);
}

template <size_t size>
double * MyContainerClass<size>::end(){
    return std::end(mContainer);
}

In other member functions, I am attempting to use begin() and end() for STL algorithms such as std::transform and std::copy. When const objects are passed as parameters to these member functions, I encounter the error:

error: passing 'const MyContainerClass<size>' as 'this' argument discards qualifiers.
note: in call to 'double* MyContainerClass<size>::begin() [with unsigned int size = size]'

Is this caused by incorrect begin() and end() implementations?

std::copy(begin(), end(), someOutputIterator);

Upvotes: 2

Views: 167

Answers (2)

Rene
Rene

Reputation: 2474

The important word here is 'const': You need to provide additional const versions of your begin() and end() functions, that return const double*.

If you are using C++ 11, you may also want to provide cbegin() and cend().

Upvotes: 4

juanchopanza
juanchopanza

Reputation: 227400

Is this caused by incorrect begin() and end() implementations?

Yes, you need const versions of the functions. For example:

template <size_t size>
const double * MyContainerClass<size>::begin() const {
    return std::begin(mContainer);
}

Upvotes: 4

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