Can't I get a pointer as a parameter of STL predicate?

Question

How can I get a pointer?

I want to put a pointer as a parameter to the lambda expression you see!

vector<unique_ptr<int>> arr{};

for (int i = 0; i < 100; ++i) {
  arr.emplace_back(i);
}

auto p = find_if(arr.begin(), arr.end(),
                 [](const unique_ptr<int>& a)  // error
                 { return *a == 10; });

cout << *p << endl;

Answer 1

The problem is that there is no implicit constructor for the class template std::unique_ptr that accepts an object of the template argument type.

So this loop

for (int i = 0; i < 100; ++i)
{
    arr.emplace_back(i);
}

is incorrect.

Also this statement

cout << *p << endl;

is also incorrect. Instead write

cout << **p << endl;

It seems you mean the following

for (int i = 0; i < 100; ++i)
{
    arr.emplace_back( make_unique<int>( i ) );
}

auto p = find_if(arr.begin(), arr.end(), [](const unique_ptr<int>& a) // error
{
    return *a == 10;
});

if ( p != arr.end() ) cout << **p << endl;

Answer 2

The problem is not the lambda, it is how you construct elements in your vector. This is how you want to do it:

for (int i = 0; i != 100; ++i) {
    arr.emplace_back( std::make_unique<int>( i ) );
}