Range-based for loop on a dynamic array?

问题:

There is a range-based for loop with the syntax:

for(auto& i : array) 

It works with constant arrays but not with pointer based dynamic ones, like

int *array = new int[size]; for(auto& i : array)    cout

It gives errors and warnings about failure of substitution, for instance:

Error] C:\Users\Siegfred\Documents\C-Free\Temp\Untitled2.cpp:16:16: error: no matching function for call to 'begin(int*&)'

How do I use this new syntax with dynamic arrays?

回答1:

To make use of the range-based for-loop you have to provide either begin() and end() member functions or overload the non-member begin() and end() functions. In the latter case, you can wrap your range in a std::pair and overload begin() and end() for those:

    namespace std {         template  T* begin(std::pair const& p)         { return p.first; }         template  T* end(std::pair const& p)         { return p.second; }     } 

Now you can use the for-loop like this:

    for (auto&& i : std::make_pair(array, array + size))         cout 

Note, that the non-member begin() and end() functions have to be overloaded in the std namespace here, because pair also resides in namespace std. If you don't feel like tampering with the standard namespace, you can simply create your own tiny pair class and overload begin() and end() in your namespace.

Or, create a thin wrapper around your dynamically allocated array and provide begin() and end() member functions:

    template      struct wrapped_array {         wrapped_array(T* first, T* last) : begin_ {first}, end_ {last} {}         wrapped_array(T* first, std::ptrdiff_t size)             : wrapped_array {first, first + size} {}          T*  begin() const noexcept { return begin_; }         T*  end() const noexcept { return end_; }          T* begin_;         T* end_;     };      template      wrapped_array wrap_array(T* first, std::ptrdiff_t size) noexcept     { return {first, size}; } 

And your call site looks like this:

    for (auto&& i : wrap_array(array, size))          std::cout 

Example

回答2:

You can't use range-for-loop with dynamically allocated arrays, since compiler can't deduce begin and end of this array. You should always use containers instead of it, for example std::vector.

std::vector v(size); for(const auto& elem: v)     // do something 

回答3:

You can't perform a range based loop over a dynamically allocated array because all you have is a pointer to the first element. There is no information concerning its size that the compiler can use to perform the loop. The idiomatic C++ solution would be to replace the dynamically allocated array by an std::vector:

std::vector arr(size); for(const auto& i : arr)   std::cout

A range based for loop does work for std::array objects. You have to look over the array instance (arr), not the type (array):

std::array arr; for(const auto& i : arr)   std::cout

回答4:

See this page http://www.codeproject.com/Articles/570638/Ten-Cplusplus11-Features-Every-Cplusplus-Developer and find the chapter "non-member begin() and end()". This could be what you want to achieve.

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