Two Questions about c++ lambda functions. [Must Read]

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Two Questions about c++ lambda functions. [Must Read]

Автор Boring_Day, история, 2 года назад, По-английски

So today i was learning about c++ lambda functions. and i have some doubts.

Q.1) See this

why the output is 2?

        function<int(int,int)> sum = [&](int a, int b)
        {
            ++a;
            return a+b;
        };
        int a = 2, b = 3;
        int s = sum(a, b);
        cout << a << endl;// why it's 2 i have passed 'a' with refrence. how to pass it with refrence.

Q.2) another question

This is giving me runtime error why?

        function<int(int)> get2;
        
        cout << get2(3);

        get2 = [&](int a)
        {
            return a;
        };

tell me

question, c++, lambda, important

Boring_Day
2 года назад
10

Комментарии (7)

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CountZero

2 года назад, # |

why it's 2 i have passed 'a' with refrence
[&](int a, int b)

a is passed by value here, not by reference. & is for captured variables, not for ones passed as arguments.

how can i use the get function?

sum and get are STL functions, don't name your variables like this. Anyway, in this case you should split declaration and implementation, just like with usual functions:

function<int(int)> sum2, get2;

sum2 = [&](int a) {
    cout << get2(2);
    return 0;
};

get2 = [&](int v) {
    int b = sum2(3);
    return v;
};

Also, try your best to avoid using std::function, this thing will hurt your program's performace and can easily lead to TLE. Use auto wherever possible

auto sum2 = [&](int a, int b) { ...

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Boring_Day

2 года назад, # ^ |

← Rev. 2 →

How to pass 'a' as reference. And i searched online about capture clause but i couldn't understand can you explain with the help of code.

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skoriop

2 года назад, # ^ |

Captured variables are variables just outside the lambda function that can be used in its body (not its arguments). They can be passed by reference [&] or by value [=].

To pass 'a' by reference, just use 'int& a' in the lambda's header.

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Boring_Day

2 года назад, # ^ |

This is giving me error can you please explain why?

Spoiler


        function<int(int)> get;

        cout << get(3);

        function<int(int)> get = [&](int a)
        {
            return a;
        };

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skoriop

2 года назад, # ^ |

In C++, you have to declare & implement the function before calling it.

This should work:

function<int(int)> get;

get = [&](int a)
{
    return a;
};

cout << get(3);

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Boring_Day

2 года назад, # ^ |

please try to understand i want to use two functions inside each other in some dp problems. how can i do this.

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skoriop

2 года назад, # ^ |

All you need is forward declaration then, like CountZero said:

Code

        function<int(int)> get;

	function<int(int)> sum = [&](int a)
	{
		cout << get(2);
		return a;
	};

	get = [&](int v)
	{
		int b = sum(3);
		return v;
	};

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