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# Opaque Containers

Normally, Python containers such as `list` or `dict` are passed when calling C++ functions taking a corresponding C++ container (see container-type).

This means that for each call, the entire Python container is converted to a C++ container, which can be inefficient when for example creating plots from lists of points.

To work around this, special opaque containers can generated which wrap an underlying C++ container directly (currently implemented for `list` types). They implement the sequence protocol and can be passed to the function instead of a Python list. Manipulations like adding or removing elements can applied directly to them using the C++ container functions.

This is achieved by specifying the name and the instantiated type in the `opaque-containers` attribute of container-type.

A second use case are public fields of container types. In the normal case, they are converted to Python containers on read access. By a field modification, (see modify-field), it is possible to obtain an opaque container which avoids the conversion and allows for direct modification of elements.

Getters returning references can also be modified to return opaque containers. This is done by modifying the return type to the name of the opaque container (see replace-type).

The table below lists the functions supported for opaque sequence containers besides the sequence protocol (element access via index and `len()`). Both the STL and the Qt naming convention (which resembles Python’s) are supported:

> Function | Description
> `push_back(value)`, `append(value)` | Appends value to the sequence.
> `push_front(value)`, `prepend(value)` | Prepends value to the sequence.
> `clear()` | Clears the sequence.
> `pop_back()`, `removeLast()` | Removes the last element.
> `pop_front()`, `removeFirst()` | Removes the first element.
> `reserve(size)` | For containers that support it (`std::vector`, `QList`), allocate memory for at least `size` elements, preventing reallocations.
> `capacity()` | For containers that support it (`std::vector`, `QList`), return the number of elements that can be stored without reallocation.