实例讲解C++编程中lambda表达式的使用
|
函数对象与Lambdas
// even_lambda.cpp
// compile with: cl /EHsc /nologo /W4 /MTd
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main()
{
// Create a vector object that contains 10 elements.
vector<int> v;
for (int i = 1; i < 10; ++i) {
v.push_back(i);
}
// Count the number of even numbers in the vector by
// using the for_each function and a lambda.
int evenCount = 0;
for_each(v.begin(),v.end(),[&evenCount] (int n) {
cout << n;
if (n % 2 == 0) {
cout << "is even" << endl;
++evenCount;
} else {
cout << "is odd" << endl;
}
});
// Print the count of even numbers to the console.
cout << "There are " << evenCount
<< " even numbers in the vector." << endl;
}
输出 1 is even 2 is odd 3 is even 4 is odd 5 is even 6 is odd 7 is even 8 is odd 9 is even There are 4 even numbers in the vector. 批注 代码
// even_functor.cpp
// compile with: /EHsc
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
class FunctorClass
{
public:
// The required constructor for this example.
explicit FunctorClass(int& evenCount)
: m_evenCount(evenCount) { }
// The function-call operator prints whether the number is
// even or odd. If the number is even,this method updates
// the counter.
void operator()(int n) const {
cout << n;
if (n % 2 == 0) {
cout << " is even " << endl;
++m_evenCount;
} else {
cout << " is odd " << endl;
}
}
private:
// Default assignment operator to silence warning C4512.
FunctorClass& operator=(const FunctorClass&);
int& m_evenCount; // the number of even variables in the vector.
};
int main()
{
// Create a vector object that contains 10 elements.
vector<int> v;
for (int i = 1; i < 10; ++i) {
v.push_back(i);
}
// Count the number of even numbers in the vector by
// using the for_each function and a function object.
int evenCount = 0;
for_each(v.begin(),FunctorClass(evenCount));
// Print the count of even numbers to the console.
cout << "There are " << evenCount
<< " even numbers in the vector." << endl;
}
输出 1 is even 2 is odd 3 is even 4 is odd 5 is even 6 is odd 7 is even 8 is odd 9 is even There are 4 even numbers in the vector.
// declaring_lambda_expressions1.cpp
// compile with: /EHsc /W4
#include <functional>
#include <iostream>
int main()
{
using namespace std;
// Assign the lambda expression that adds two numbers to an auto variable.
auto f1 = [](int x,int y) { return x + y; };
cout << f1(2,3) << endl;
// Assign the same lambda expression to a function object.
function<int(int,int)> f2 = [](int x,int y) { return x + y; };
cout << f2(3,4) << endl;
}
输出 5 7 备注
// declaring_lambda_expressions2.cpp
// compile with: /EHsc /W4
#include <functional>
#include <iostream>
int main()
{
using namespace std;
int i = 3;
int j = 5;
// The following lambda expression captures i by value and
// j by reference.
function<int (void)> f = [i,&j] { return i + j; };
// Change the values of i and j.
i = 22;
j = 44;
// Call f and print its result.
cout << f() << endl;
}
输出 47 调用 Lambda 表达式
// calling_lambda_expressions1.cpp
// compile with: /EHsc
#include <iostream>
int main()
{
using namespace std;
int n = [] (int x,int y) { return x + y; }(5,4);
cout << n << endl;
}
输出 复制代码 代码如下: 9
示例 2
// calling_lambda_expressions2.cpp
// compile with: /EHsc /W4
#include <list>
#include <algorithm>
#include <iostream>
int main()
{
using namespace std;
// Create a list of integers with a few initial elements.
list<int> numbers;
numbers.push_back(13);
numbers.push_back(17);
numbers.push_back(42);
numbers.push_back(46);
numbers.push_back(99);
// Use the find_if function and a lambda expression to find the
// first even number in the list.
const list<int>::const_iterator result =
find_if(numbers.begin(),numbers.end(),[](int n) { return (n % 2) == 0; });
// Print the result.
if (result != numbers.end()) {
cout << "The first even number in the list is " << *result << "." << endl;
} else {
cout << "The list contains no even numbers." << endl;
}
}
输出 The first even number in the list is 42. 嵌套 Lambda 表达式
// nesting_lambda_expressions.cpp
// compile with: /EHsc /W4
#include <iostream>
int main()
{
using namespace std;
// The following lambda expression contains a nested lambda
// expression.
int timestwoplusthree = [](int x) { return [](int y) { return y * 2; }(x) + 3; }(5);
// Print the result.
cout << timestwoplusthree << endl;
}
输出 复制代码 代码如下: 13
备注
在该示例中，[](int y) { return y * 2; } 是嵌套的 lambda 表达式。 高阶 Lambda 函数 示例 许多编程语言都支持高阶函数的概念。 高阶函数是采用另一个 lambda 表达式作为其参数或返回 lambda 表达式的 lambda 表达式。你可以使用 function 类，使得 C++ lambda 表达式具有类似高阶函数的行为。以下示例显示返回 function 对象的 lambda 表达式和采用 function 对象作为其参数的 lambda 表达式。 代码
// higher_order_lambda_expression.cpp
// compile with: /EHsc /W4
#include <iostream>
#include <functional>
int main()
{
using namespace std;
// The following code declares a lambda expression that returns
// another lambda expression that adds two numbers.
// The returned lambda expression captures parameter x by value.
auto addtwointegers = [](int x) -> function<int(int)> {
return [=](int y) { return x + y; };
};
// The following code declares a lambda expression that takes another
// lambda expression as its argument.
// The lambda expression applies the argument z to the function f
// and multiplies by 2.
auto higherorder = [](const function<int(int)>& f,int z) {
return f(z) * 2;
};
// Call the lambda expression that is bound to higherorder.
auto answer = higherorder(addtwointegers(7),8);
// Print the result,which is (7+8)*2.
cout << answer << endl;
}
输出 复制代码 代码如下: 30
在函数中使用 Lambda 表达式
示例 你可以在函数的主体中使用 lambda 表达式。lambda 表达式可以访问该封闭函数可访问的任何函数或数据成员。你可以显式或隐式捕获 this 指针，以提供对封闭类的函数和数据成员的访问路径。 你可以在函数中显式使用 this 指针，如下所示：
void ApplyScale(const vector<int>& v) const
{
for_each(v.begin(),[this](int n) { cout << n * _scale << endl; });
}
你也可以隐式捕获 this 指针：
void ApplyScale(const vector<int>& v) const
{
for_each(v.begin(),[=](int n) { cout << n * _scale << endl; });
}
以下示例显示封装小数位数值的 Scale 类。
// function_lambda_expression.cpp
// compile with: /EHsc /W4
#include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
class Scale
{
public:
// The constructor.
explicit Scale(int scale) : _scale(scale) {}
// Prints the product of each element in a vector object
// and the scale value to the console.
void ApplyScale(const vector<int>& v) const
{
for_each(v.begin(),[=](int n) { cout << n * _scale << endl; });
}
private:
int _scale;
};
int main()
{
vector<int> values;
values.push_back(1);
values.push_back(2);
values.push_back(3);
values.push_back(4);
// Create a Scale object that scales elements by 3 and apply
// it to the vector object. Does not modify the vector.
Scale s(3);
s.ApplyScale(values);
}
输出 3 6 9 12 备注
// template_lambda_expression.cpp
// compile with: /EHsc
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;
// Negates each element in the vector object. Assumes signed data type.
template <typename T>
void negate_all(vector<T>& v)
{
for_each(v.begin(),[](T& n) { n = -n; });
}
// Prints to the console each element in the vector object.
template <typename T>
void print_all(const vector<T>& v)
{
for_each(v.begin(),[](const T& n) { cout << n << endl; });
}
int main()
{
// Create a vector of signed integers with a few elements.
vector<int> v;
v.push_back(34);
v.push_back(-43);
v.push_back(56);
print_all(v);
negate_all(v);
cout << "After negate_all():" << endl;
print_all(v);
}
输出 34 -43 56 After negate_all(): -34 43 -56 处理异常
// eh_lambda_expression.cpp
// compile with: /EHsc /W4
#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;
int main()
{
// Create a vector that contains 3 elements.
vector<int> elements(3);
// Create another vector that contains index values.
vector<int> indices(3);
indices[0] = 0;
indices[1] = -1; // This is not a valid subscript. It will trigger an exception.
indices[2] = 2;
// Use the values from the vector of index values to
// fill the elements vector. This example uses a
// try/catch block to handle invalid access to the
// elements vector.
try
{
for_each(indices.begin(),indices.end(),[&](int index) {
elements.at(index) = index;
});
}
catch (const out_of_range& e)
{
cerr << "Caught '" << e.what() << "'." << endl;
};
}
输出 Caught 'invalid vector<T> subscript'. 备注 配合使用 Lambda 表达式和托管类型 (C++/CLI)
// managed_lambda_expression.cpp
// compile with: /clr
using namespace System;
int main()
{
char ch = '!'; // a local unmanaged variable
// The following lambda expression captures local variables
// by value and takes a managed String object as its parameter.
[=](String ^s) {
Console::WriteLine(s + Convert::ToChar(ch));
}("Hello");
}
输出 Hello! （编辑：李大同） 【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容! |