C++ std::find_if

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本文详细介绍了 C++ 中的 find_if 函数，该函数用于查找满足特定条件的第一个元素。文章提供了 find_if 的参数说明、返回值以及一个示例程序，展示了如何使用 find_if 查找向量中第一个奇数。

template <class InputIterator, class UnaryPredicate>

Input Iterator find_if (InputIterator first, InputIterator last, UnaryPredicate pred);

Find element in range
Returns an iterator to the first element in the range [first,last) for which pred returns true. If no such element is found, the function returns last.

The behavior of this function template is equivalent to:

template<class InputIterator, class UnaryPredicate>
  InputIterator find_if (InputIterator first, InputIterator last, UnaryPredicate pred)
{
  while (first!=last) {
    if (pred(*first)) return first;
    ++first;
  }
  return last;
}
template<class InputIterator, class UnaryPredicate>
  InputIterator find_if (InputIterator first, InputIterator last, UnaryPredicate pred)
{
  while (first!=last) {
    if (pred(*first)) return first;
    ++first;
  }
  return last;
}

Parameters

first, last

Input iterators to the initial and final positions in a sequence. The range used is [first,last), which contains all the elements between first and last, including the element pointed by first but not the element pointed by last.

pred

Unary function that accepts an element in the range as argument and returns a value convertible to bool. The value returned indicates whether the element is considered a match in the context of this function.
The function shall not modify its argument.
This can either be a function pointer or a function object.

Return value

An iterator to the first element in the range for which pred does not return false.
If pred is false for all elements, the function returns last.

Example

// find_if example
#include <iostream>     // std::cout
#include <algorithm>    // std::find_if
#include <vector>       // std::vector

bool IsOdd (int i) {
  return ((i%2)==1);
}

int main () {
  std::vector<int> myvector;

  myvector.push_back(10);
  myvector.push_back(25);
  myvector.push_back(40);
  myvector.push_back(55);

  std::vector<int>::iterator it = std::find_if (myvector.begin(), myvector.end(), IsOdd);
  std::cout << "The first odd value is " << *it << '\n';

  return 0;
}