CODEWITHSUNDEEP
c++lambda
Bupa
Bupa

Reputation: 247

C++ lambda copy beahaviour

I wanted to undersatnd variable i scope w.r.to lambda function. i is captured as by value So incrementation should only happen internally and it should not effect the global i value. So i expected output 1 1 1. But the output is 1 2 2

    int i = 0;
    auto a = [i]() mutable { cout << ++i << endl; };
    a(); // i copies by value , So i value get changed internally in lambda function scope
    auto b = a;
    a(); // here why it is giving 2 instead of 1 ???
    b();

What am i missing here ?

Upvotes: 1

Views: 2084

Answers (2)

Swift - Friday Pie
Swift - Friday Pie

Reputation: 14614

Result of lambda expression is an object with operator(). Code here

int i = 0;
auto a = [i]() mutable { cout << ++i << endl; };
a();
a();

results in behaviour roughly similar to this:

int i = 0;
struct local_lambda_01 {
private: // some older compilers skip this and in result there is an 
         // implementation detail that you can access following members
    int i = ::i;   // this is result of capture by value.
public:
    void operator() {  // it's not `const` because lambda is `mutable`
         cout << ++i << endl;  // it's `this->i`
    }    
} a;   // a variable of local_lambda_01  type;

a();  // call to operator(), increments member i of local_lambda_01 
a();

A copy of variable i's value was stored at point of creation of callable a instance. As lambda was declared mutable, operator() is not const-declared and can modify stored values. It doesn't change the original.

If we add statement

auto b = a;

after calling a(), we perform copy on already modified instance of the object.

Upvotes: 2

songyuanyao
songyuanyao

Reputation: 172934

int i = 0;
auto a = [i]() mutable { cout << ++i << endl; }; // i is captured in a with value 0
a();        // i captured by a becomes 1
auto b = a; // b is copied from a, the captured i in b is copied with value 1 too
a();        // i captured by a becomes 2
b();        // i captured by b becomes 2

Upvotes: 1

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