What are the pitfalls to be aware of when using STL function objects?

STL function object usage trap: The state of the function object cannot be modified, otherwise it may cause consequences or crash. Function objects should be used as rvalues, lvalue use causes undefined behavior. When capturing local variables you should be sure to capture all referenced variables, otherwise a crash may result.

Pitfalls to be aware of when using STL function objects

STL function objects are a powerful tool that can simplify code and improve readability. However, you need to be careful when using them as there are some potential pitfalls to be aware of.

Trap 1: Do not modify the state of the function object

The state of the function object should be immutable. If you try to modify the internal state of a function object, you may have unexpected consequences or even crash.

// 错误示范
auto f = []() {
  static int x = 0;  // 可变状态
  return ++x;  // 修改可变状态
};

Trap 2: Don’t use function objects as lvalues

Function objects should always be used as rvalues. Undefined behavior results if you use a function object as an lvalue.

// 错误示范
auto f = []() { return 42; };
f = []() { return 99; };  // 将函数对象作为左值使用

Trap 3: Do not capture different variables at the same time

When capturing local variables, be sure to capture all referenced variables, otherwise the program may crash.

// 错误示范
struct Foo {
  int& x;

  Foo(int& x) : x(x) {}
  int operator()() const { return x; }
};

Practical case

Consider the following example, which uses the STL function object std::find to find the first match in a given container Position of elements:

#include <vector>
#include <algorithm>

int main() {
  std::vector<int> v = {1, 2, 3, 4, 5};
  auto it = std::find(v.begin(), v.end(), 3);
  if (it != v.end()) {
    std::cout << "找到元素 3" << std::endl;
  }

  return 0;
}

Following these pitfalls and using STL function objects carefully will help avoid unexpected behavior and write clean, reliable code.

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