(use_cases_howto_py_pass_reference_py)=
# Macro **"howtoPassReference.py"**

## Objective

The goal of this macro is to show how to use function that would have been defined using the 
"by-reference" prototype, which is specific to C++. A simple exemple in {{uranie}} is provided when 
considering the `computeQuantile` method: the second argument is a double in which the result will 
be stored.

## Macro {{uranie}}

{{
    "```{literalinclude} " + parent_dir +
        "/roottest/uranie/doc/overview/use_cases/python/howtoPassReference.py\n"
    + ":language: python\n"
    + ":lines: 1-5,11-23\n"
    + "```"
}}

It starts by calling the `ctypes` module in order to get the `c_double` interface. This is only for 
{{root}} version greater than 6.20. For older version, the solution relied on {{root}} only (calling 
another interface discussed later-on).

{{
    "```{literalinclude} " + parent_dir +
        "/roottest/uranie/doc/overview/use_cases/python/howtoPassReference.py\n"
    + ":language: python\n"
    + ":lines: 4\n"
    + "```"
}}

Once done, a {{tds}} object is created and filled from the `myData.dat` file. One then wants to 
estimate the 90% quantile, which can be done by calling the `computeQuantile` method, which requires 
two double as input, both passed "by-reference". To do so, one has to create two specific objects as 
below:

{{
    "```{literalinclude} " + parent_dir +
        "/roottest/uranie/doc/overview/use_cases/python/howtoPassReference.py\n"
    + ":language: python\n"
    + ":lines: 17-18\n"
    + "```"
}}

Once done, the method is called and the result is displayed by getting the value of the `c_double` 
object

{{
    "```{literalinclude} " + parent_dir +
        "/roottest/uranie/doc/overview/use_cases/python/howtoPassReference.py\n"
    + ":language: python\n"
    + ":lines: 23\n"
    + "```"
}}

The output of this macro is shown in [](#use_cases_howto_py_pass_reference_py_console).

(use_cases_howto_py_pass_reference_py_console)=
## Console

{{
    "```{literalinclude} " + parent_dir +
        "/roottest/build/uranie/doc/overview/use_cases/python/howtoPassReference_clean.log\n"
    + ":language: none\n"
    + ":lines: 2-8\n"
    + "```"
}}