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      std::fill

      From cppreference.com
      <cpp‎ |algorithm
       
       
      Algorithm library
      Constrained algorithms and algorithms on ranges(C++20)
      Constrained algorithms, e.g.ranges::copy,ranges::sort, ...
      Execution policies(C++17)
      Sorting and related operations
      Partitioning operations
      Sorting operations
      Binary search operations
      (on partitioned ranges)
      Set operations (on sorted ranges)
      Merge operations (on sorted ranges)
      Heap operations
      Minimum/maximum operations
      (C++11)
      (C++17)
      Lexicographical comparison operations
      Permutation operations
      C library
      Numeric operations
      Operations on uninitialized memory
       
      Defined in header<algorithm>
      (1)
      template<class ForwardIt,class T>
      void fill( ForwardIt first, ForwardIt last,const T& value);
      (constexpr since C++20)
      (until C++26)
      template<class ForwardIt,class T=typenamestd::iterator_traits

                                               <ForwardIt>::value_type>
      constexprvoid fill( ForwardIt first, ForwardIt last,

                           const T& value);
      (since C++26)
      (2)
      template<class ExecutionPolicy,class ForwardIt,class T>

      void fill( ExecutionPolicy&& policy,

                 ForwardIt first, ForwardIt last,const T& value);
      (since C++17)
      (until C++26)
      template<class ExecutionPolicy,

               class ForwardIt,class T=typenamestd::iterator_traits
                                               <ForwardIt>::value_type>
      void fill( ExecutionPolicy&& policy,

                 ForwardIt first, ForwardIt last,const T& value);
      (since C++26)
      1) Assigns the givenvalue to all elements in the range[firstlast).
      2) Same as(1), but executed according topolicy.
      This overload participates in overload resolution only if all following conditions are satisfied:

      std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> istrue.

      (until C++20)

      std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> istrue.

      (since C++20)

      Ifvalue is notwritable tofirst, the program is ill-formed.

      Contents

      [edit]Parameters

      first, last - the pair of iterators defining therange of elements to modify
      value - the value to be assigned
      policy - theexecution policy to use
      Type requirements
      -
      ForwardIt must meet the requirements ofLegacyForwardIterator.

      [edit]Complexity

      Exactlystd::distance(first, last) assignments.

      [edit]Exceptions

      The overload with a template parameter namedExecutionPolicy reports errors as follows:

      • If execution of a function invoked as part of the algorithm throws an exception andExecutionPolicy is one of thestandard policies,std::terminate is called. For any otherExecutionPolicy, the behavior is implementation-defined.
      • If the algorithm fails to allocate memory,std::bad_alloc is thrown.

      [edit]Possible implementation

      fill (1)
      template<class ForwardIt,class T=typenamestd::iterator_traits<ForwardIt>::value_type>void fill(ForwardIt first, ForwardIt last,const T& value){for(; first!= last;++first)*first= value;}

      [edit]Notes

      Feature-test macroValueStdFeature
      __cpp_lib_algorithm_default_value_type202403(C++26)List-initialization for algorithms(1,2)

      [edit]Example

      Run this code
      #include <algorithm>#include <complex>#include <iostream>#include <vector> void println(constauto& seq){for(constauto& e: seq)std::cout<< e<<' ';std::cout<<'\n';} int main(){std::vector<int> v{0,1,2,3,4,5,6,7,8};    println(v); // set all of the elements to 8    std::fill(v.begin(), v.end(),8);    println(v); std::vector<std::complex<double>> nums{{1,3},{2,2},{4,8}};    println(nums);#ifdef __cpp_lib_algorithm_default_value_type        std::fill(nums.begin(), nums.end(),{4,2});#else        std::fill(nums.begin(), nums.end(),std::complex<double>{4,2});#endif    println(nums);}

      Output:

      0 1 2 3 4 5 6 7 88 8 8 8 8 8 8 8 8(1,3) (2,2) (4,8) (4,2) (4,2) (4,2)

      [edit]Defect reports

      The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

      DRApplied toBehavior as publishedCorrect behavior
      LWG 283C++98T was required to beCopyAssignable, but
      T is not always writable toForwardIt
      required to be writable instead

      [edit]See also

      copy-assigns the given value to N elements in a range
      (function template)[edit]
      copies a range of elements to a new location
      (function template)[edit]
      assigns the results of successive function calls to every element in a range
      (function template)[edit]
      applies a function to a range of elements, storing results in a destination range
      (function template)[edit]
      assigns a range of elements a certain value
      (algorithm function object)[edit]
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