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std::generate

From cppreference.com
< cpp‎ | algorithm
 
 
Algorithm library
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Operations on uninitialized storage
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
C library
 
Defined in header <algorithm>
template< class ForwardIt, class Generator >
void generate( ForwardIt first, ForwardIt last, Generator g );
(1)
template< class ExecutionPolicy, class ForwardIt, class Generator >
void generate( ExecutionPolicy&& policy, ForwardIt first, ForwardIt last, Generator g );
(2) (since C++17)
1) Assigns each element in range [first, last) a value generated by the given function object g.
2) Same as (1), but executed according to policy. This overload does not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

Contents

[edit] Parameters

first, last - the range of elements to generate
policy - the execution policy to use. See execution policy for details.
g - generator function object that will be called.

The signature of the function should be equivalent to the following:

Ret fun();

The type Ret must be such that an object of type ForwardIt can be dereferenced and assigned a value of type Ret. ​

Type requirements
-
ForwardIt must meet the requirements of ForwardIterator.

[edit] Return value

(none)

[edit] Complexity

Exactly std::distance(first, last) invocations of g() and assignments.

[edit] Exceptions

The overload with a template parameter named ExecutionPolicy reports errors as follows:

  • If execution of a function invoked as part of the algorithm throws an exception,
  • if policy is std::parallel_vector_execution_policy, std::terminate is called
  • if policy is std::sequential_execution_policy or std::parallel_execution_policy, the algorithm exits with an std::exception_list containing all uncaught exceptions. If there was only one uncaught exception, the algorithm may rethrow it without wrapping in std::exception_list. It is unspecified how much work the algorithm will perform before returning after the first exception was encountered.
  • if policy is some other type, the behavior is implementation-defined
  • If the algorithm fails to allocate memory (either for itself or to construct an std::exception_list when handling a user exception), std::bad_alloc is thrown.

[edit] Possible implementation

template<class ForwardIt, class Generator>
void generate(ForwardIt first, ForwardIt last, Generator g)
{
    while (first != last) {
        *first++ = g();
    }
}

[edit] Example

The following code fills a vector with random numbers:

#include <algorithm>
#include <iostream>
#include <vector>
#include <cstdlib>
 
int main()
{
    std::vector<int> v(5);
    std::generate(v.begin(), v.end(), std::rand); // Using the C function rand()
 
    std::cout << "v: ";
    for (auto iv: v) {
        std::cout << iv << " ";
    }
    std::cout << "\n";
 
    // Initialize with default values 0,1,2,3,4 from a lambda function
    // Equivalent to std::iota(v.begin(), v.end(), 0);
    int n = {0};
    std::generate(v.begin(), v.end(), [&n]{ return n++; });
 
    std::cout << "v: ";
    for (auto iv: v) {
        std::cout << iv << " ";
    }
    std::cout << "\n";
}

Possible output:

v: 52894 15984720 41513563 41346135 51451456
v: 0 1 2 3 4

[edit] See also

assigns a range of elements a certain value
(function template) [edit]
saves the result of N applications of a function
(function template) [edit]
parallelized version of std::generate
(function template) [edit]