Why are these constructors of std::stack explicit

Question

I have questions about std::stack Why these two constructors are explicit ?

     explicit stack( const Container& cont = Container() );
     explicit stack( Container&& cont = Container() );

Note: Source

I don't get your 3. (http://en.cppreference.com/w/cpp/container/stack/operator%3D)? — Holt, Jun 29 '16 at 14:27
(1): [Possible duplicate](http://stackoverflow.com/questions/102459/why-does-stdstack-use-stddeque-by-default?rq=1) — sjrowlinson, Jun 29 '16 at 14:28
Why don't you just ask question 2 so we do not have to close this as a duplicate of: http://stackoverflow.com/questions/102459/why-does-stdstack-use-stddeque-by-default?rq=1 — NathanOliver, Jun 29 '16 at 14:29
You wouldn't want a function expecting a `const std::stack&` to accept an `std::vector` and create a temporary copy every time. The `explicit` is there to warn the caller that they should probably handle the conversion, either with an explicit cast or by creating an `std::stack` — KABoissonneault, Jun 29 '16 at 14:48
@Hurkyl surely the more interesting interpretation of the question is "why should implicit construction from these constructors be avoided". — eerorika, Jun 29 '16 at 14:50

score 4 · Answer 1 · answered Jun 29 '16 at 15:12

4

The constructors are explicit so you can't accidentally pass an underlying container (such as vector or deque) to a function expecting a stack, resulting in unexpected copying (not to mention violating the principle of least surprise).

answered Jun 29 '16 at 15:12

Mark B

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Curious · Answer 2 · 2016-06-29T15:14:00.783

One problem is that if you assume implicit invocation, what would happen if others followed your example? So for instance the following fails to compile

#include <iostream>
#include <vector>
using namespace std;

class Test {
  public:

    Test(const std::vector<int>&) {
        cout << "Test(const std::Vector<int>&)" << endl;
    }
};
class AnotherTest {
  public:

    AnotherTest(const std::vector<int>&) {
        cout << "AnotherTest(const std::Vector<int>&)" << endl;
    }
};

void test_function(const AnotherTest&) {
    cout << "fucntion(const AnotherTest&)" << endl;
}
void test_function(const Test&) {
    cout << "fucntion(const Test&)" << endl;
}

int main() {
    const std::vector<int> vec {1, 2, 3};
    test_function(vec);

    return 0;
}

You can easily see this problem occurring with a stack and a queue.

Why are these constructors of std::stack explicit

2 Answers2