# Extra Clang Tools 4 documentation

## clang-tidy - modernize-use-emplace

# modernize-use-emplace

The check flags insertions to an STL-style container done by calling the `push_back` method with an explicitly-constructed temporary of the container element type. In this case, the corresponding `emplace_back` method results in less verbose and potentially more efficient code. Right now the check doesn’t support `push_front` and `insert`. It also doesn’t support `insert` functions for associative containers because replacing `insert` with `emplace` may result in speed regression, but it might get support with some addition flag in the future.

By default only `std::vector`, `std::deque`, `std::list` are considered. This list can be modified using the `ContainersWithPushBack` option.

Before:

    std::vector<MyClass> v;
    v.push_back(MyClass(21, 37));

    std::vector<std::pair<int, int>> w;

    w.push_back(std::pair<int, int>(21, 37));
    w.push_back(std::make_pair(21L, 37L));

After:

    std::vector<MyClass> v;
    v.emplace_back(21, 37);

    std::vector<std::pair<int, int>> w;
    w.emplace_back(21, 37);
    // This will be fixed to w.push_back(21, 37); in next version
    w.emplace_back(std::make_pair(21L, 37L);

The other situation is when we pass arguments that will be converted to a type inside a container.

Before:

    std::vector<boost::optional<std::string> > v;
    v.push_back("abc");

After:

    std::vector<boost::optional<std::string> > v;
    v.emplace_back("abc");

In some cases the transformation would be valid, but the code wouldn’t be exception safe. In this case the calls of `push_back` won’t be replaced.

    std::vector<std::unique_ptr<int>> v;
    v.push_back(std::unique_ptr<int>(new int(0)));
    auto *ptr = new int(1);
    v.push_back(std::unique_ptr<int>(ptr));

This is because replacing it with `emplace_back` could cause a leak of this pointer if `emplace_back` would throw exception before emplacement (e.g. not enough memory to add new element).

For more info read item 42 - “Consider emplacement instead of insertion.” of Scott Meyers “Effective Modern C++”.

The default smart pointers that are considered are `std::unique_ptr`, `std::shared_ptr`, `std::auto_ptr`. To specify other smart pointers or other classes use the `SmartPointers` option.

Check also fires if any argument of constructor call would be:

> - bitfield (bitfields can’t bind to rvalue/universal reference)
> - `new` expression (to avoid leak) or if the argument would be converted via derived-to-base cast.

This check requires C++11 of higher to run.

## Options

`ContainersWithPushBack```

  Semicolon-separated list of class names of custom containers that support `push_back`.

`SmartPointers```

  Semicolon-separated list of class names of custom smart pointers.