How to use STL function objects for data validation and transformation?

Using STL function objects simplifies data validation and transformation. The verification function object returns a Boolean value indicating whether the data is valid; the conversion function object returns a new value. These function objects can be applied to data containers to perform data validation and transformations, such as validating that a number is greater than 10 and doubling numbers greater than 10.

Use STL function objects for data verification and conversion

The STL library contains a set of function objects that can perform data validation on data containers. Various operations and transformations. These function objects are very useful for handling data validation and transformation tasks concisely and efficiently.

Introduction to Function Objects

A function object is a class or structure that can be called from other functions like a normal function. They have operator overloading that allows application to data using function call syntax.

Verification function object

• ##unary_function: This function object accepts one parameter and returns a Boolean value, indicating Whether the input data is valid. For example:

struct IsEven {
    bool operator()(int x) {
        return x % 2 == 0;
    }
};

• binary_function: This function object accepts two parameters and returns a Boolean value, indicating whether the input data is valid. For example:

struct IsInVector {
    bool operator()(int x, vector<int>& v) {
        return find(v.begin(), v.end(), x) != v.end();
    }
};

Convert function object

##unary_function
• : This function object accepts one parameter and Return a new value. For example:

  struct DoubleValue {
      double operator()(int x) {
          return (double)x * 2;
      }
  };

binary_function
• : This function object accepts two parameters and returns a new value. For example:

  struct AddVectors {
      vector<int> operator()(vector<int>& v1, vector<int>& v2) {
          vector<int> result;
          for (int i = 0; i < v1.size(); i++) {
              result.push_back(v1[i] + v2[i]);
          }
          return result;
      }
  };

Practical case: Verify and convert numeric vector

Consider the following vector, you need to verify whether the number is greater than 10 and double the number greater than 10:

vector<int> numbers = {5, 12, 3, 18, 6};

You can use STL function objects for validation and conversion as follows:

// 验证是否大于 10
bool is_greater_than_10(int x) {
    return x > 10;
}

// 加倍大于 10 的数字
double double_if_greater_than_10(int x) {
    return x > 10 ? x * 2 : x;
}

// 验证并对向量应用转换
vector<int> result;
transform(numbers.begin(), numbers.end(), back_inserter(result), double_if_greater_than_10);

Now, the

result

vector will contain the converted values, where numbers greater than 10 are doubled, and Numbers less than or equal to 10 remain unchanged:

// 输出转换后的结果
for (int num : result) {
    cout << num << " ";
}

// 输出：5 24 3 36 6

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