模板函数的参数类型不能通过表达式推导

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I want to implement a container which can accept initial list of elements by supplying the constructor with sequence of values.

  9 template <typename C, int max> 10 struct c_array 11 { 12     typedef C value_type; 13  14     typedef C* iterator; 15     typedef const C* const_iterator; 16  17     typedef C& reference; 18     typedef const C& const_reference; 19  20     c_array() { } 21     template <class T> 22         c_array(typename T::iterator begin,typename T::iterator end) 23         { 24         } 25  26     C v[max]; 27     operator C*() { return v; } 28  29     reference operator[] (ptrdiff_t i) { return v[i]; } 30     const_reference operator[] (ptrdiff_t i) const { return v[i]; } 31  32     iterator begin() { return v; } 33     const_iterator begin() const { return v; } 34  35     iterator end() { return v+max; } 36     const_iterator end() const { return v+max; } 37  38     size_t size() const { return max; } 39 }; 40  41 int main(int argc, char** argv) 42 {  43     std::vector<int> myvector(10,10); 44     c_array<int,10> myarray1(myvector.begin(),myvector.end());        ... 

I am getting the following error while compiling

 ... test.cc:56:61: error: no matching function for call to ‘c_array<int, 10>::c_array(std::vector<int>::iterator, std::vector<int>::iterator)’ test.cc:56:61: note: candidates are: test.cc:22:9: note: template<class T> c_array::c_array(typename T::iterator, typename T::iterator) test.cc:22:9: note:   template argument deduction/substitution failed: test.cc:56:61: note:   couldn't deduce template parameter ‘T’ ...

Thanks in advance,

c++ templates

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asked Jun 23 '13 at 21:14

Arun Khetarpal
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1 Answer

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up vote5down voteaccepted

In the declaration

template <class T>c_array(typename T::iterator begin,typename T::iterator end)

T is used in a way called a "non-deduced context". There's no reasonable way, if you pass in arguments of type MyCustomIter, for the compiler to guess what type T might contain a typedef MyCustomIter iterator;. So the C++ Standard says compilers shouldn't even try.

Instead you can just write the function to take any type and assume/document that it should be an iterator.

template <class InputIter>c_array(InputIter begin, InputIter end) {    std::copy_n(begin, max, v);}

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answered Jun 23 '13 at 21:27

aschepler
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+1 for clearly explaining why you need that change. An additional note that may or may not be new to the OP: for ordinary functions, it isn't necessarily be a problem if template deduction cannot succeed, as long it's possible to explicitly specify the template arguments. No such syntax exists for templated constructors, so a templated constructor where the template arguments cannot be deduced will never be callable. –  hvd Jun 23 '13 at 21:44

2down votefavorite

I want to implement a container which can accept initial list of elements by supplying the constructor with sequence of values.

  9 template <typename C, int max> 10 struct c_array 11 { 12     typedef C value_type; 13  14     typedef C* iterator; 15     typedef const C* const_iterator; 16  17     typedef C& reference; 18     typedef const C& const_reference; 19  20     c_array() { } 21     template <class T> 22         c_array(typename T::iterator begin,typename T::iterator end) 23         { 24         } 25  26     C v[max]; 27     operator C*() { return v; } 28  29     reference operator[] (ptrdiff_t i) { return v[i]; } 30     const_reference operator[] (ptrdiff_t i) const { return v[i]; } 31  32     iterator begin() { return v; } 33     const_iterator begin() const { return v; } 34  35     iterator end() { return v+max; } 36     const_iterator end() const { return v+max; } 37  38     size_t size() const { return max; } 39 }; 40  41 int main(int argc, char** argv) 42 {  43     std::vector<int> myvector(10,10); 44     c_array<int,10> myarray1(myvector.begin(),myvector.end());        ... 

I am getting the following error while compiling

 ... test.cc:56:61: error: no matching function for call to ‘c_array<int, 10>::c_array(std::vector<int>::iterator, std::vector<int>::iterator)’ test.cc:56:61: note: candidates are: test.cc:22:9: note: template<class T> c_array::c_array(typename T::iterator, typename T::iterator) test.cc:22:9: note:   template argument deduction/substitution failed: test.cc:56:61: note:   couldn't deduce template parameter ‘T’ ...

Thanks in advance,

c++ templates

shareimprove this question

asked Jun 23 '13 at 21:14

Arun Khetarpal
776

 

add a comment

1 Answer

activeoldestvotes

up vote5down voteaccepted

In the declaration

template <class T>c_array(typename T::iterator begin,typename T::iterator end)

T is used in a way called a "non-deduced context". There's no reasonable way, if you pass in arguments of type MyCustomIter, for the compiler to guess what type T might contain a typedef MyCustomIter iterator;. So the C++ Standard says compilers shouldn't even try.

Instead you can just write the function to take any type and assume/document that it should be an iterator.

template <class InputIter>c_array(InputIter begin, InputIter end) {    std::copy_n(begin, max, v);}

shareimprove this answer

answered Jun 23 '13 at 21:27

aschepler
25.4k22469

 

   

+1 for clearly explaining why you need that change. An additional note that may or may not be new to the OP: for ordinary functions, it isn't necessarily be a problem if template deduction cannot succeed, as long it's possible to explicitly specify the template arguments. No such syntax exists for templated constructors, so a templated constructor where the template arguments cannot be deduced will never be callable. –  hvd Jun 23 '13 at 21:44