What's the difference between this function and the lambda?

Question

#include<iostream>

using namespace std;

int* New()
{
    return new int(666);
}
int Foo()
{
    //method1:
    int* it = New();
    return *it;
    //method2:
    return []() { return *(new int(666)); };//Complier has a complain here
    /*Both New() and Lambda are callable object, are there any differences between method1 and method2?*/ 
}
int main()
{
    cout << Foo() << endl;
    return 0;
}

I'm fresh in C++, I encounter a situation above, I had review the chapter 10.3.2 to 10.3.3 of C++ Primer, where introduces the lambda expression.But it doesn't work for me, I'm also confused about the last annotation I list.

You called New. You didn't call the lambda. Also, C is not C++. — user2357112
– user2357112, Commented Apr 2, 2017 at 2:24
Here what I mean is I will wipe off the method1 if I select method. Could you give me more some details? — Chase07
– Chase07, Commented Apr 2, 2017 at 2:34
@Ryan, compiler give me a error is : there no such convertable funtion from "lambda []int ()->int" to "int" — Chase07
– Chase07, Commented Apr 2, 2017 at 2:36

Wyzard · Accepted Answer · 2017-04-02 03:00:45Z

2

return []() { return *(new int(666)); };

This line is trying to return the lambda itself. You want to call the lambda and return the integer that it produces:

return []() { return *(new int(666)); }();  // Note the () at the end

There's generally not much point in defining a lambda function only to immediately call it, though. They're more commonly used when you need to actually return a function, or take one as an argument. (This is a more advanced thing to do, though, so you probably shouldn't worry about it for now.)

On a separate note: your program allocates integers with new, but it never releases them with delete. This is a memory leak, which is something you should avoid.

answered Apr 2, 2017 at 3:00

Wyzard

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1 Comment

Chase07 Over a year ago

Thanks a lot, I just write this small instance in haste， and will care more by ur note.

Chase07 · Accepted Answer · 2017-04-02 03:40:31Z

0

Actuall, I didn't call my lambda because it lacks the call operator. So, I fix it as:

return []() { return *(new int(666)); }();

It works now.

I review the words from Chapter of 10.3.2 of C++ Primer "We call a lambda the same way we call funtion by using the call operator".

edited Apr 2, 2017 at 3:40

answered Apr 2, 2017 at 3:21

Chase07

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