The Basics

No need for main()¶

Yay! 🎊

Displaying rich content¶

To display an object in the front-end you can omit the last semicolon of a code cell. By doing so, the last expression will be displayed:

int i = 10;
i

Declaration shadowing¶

With cling you can redefine a function, variable, or class whose definition was already provided for a particular interpreter session. That’s a big quality-of-life improvement for interpreted C++ because without it, for instance, if you start with this in a code cell:

double foo(double a)
{
    return a * a;
}

And later want to change foo() to the following, you’ll get redefinition error:

double foo(double a)
{
    return a * a * a;
}

Support for redefinitions is automatically enabled for Jupyter notebooks. You can manually turn it on/off as follows:

gClingOpts->AllowRedefinition = 1; // or 0 to disable

Using third-party libraries¶

When building a binary, you usually specify the include directories and the library path of third-party libraries in the build tool. The library will be loaded upon binary execution.

Xeus-Cling is slightly different, it allows you to specify both include directories and library path, however you need to load the library explicitly. This is done with special pragma commands that you can use in a code cell in a Jupyter Notebook:

#pragma cling add_include_path("include/directory")
#pragma cling add_library_path("lib/directory")
#pragma cling load("libname")

Magic commands¶

Magics are special commands for the kernel that are not part of the C++ programming language.

They are defined with the symbol % for a line magic and %% for a cell magic.

%%executable¶

Dump the code from all entered cells into an executable binary. The content of the cell is used for the body of the main function.

%%executable filename [-- linker options]

Example:

#include <iostream>

int square(int x) { return x * x; }

%%executable square.x
std::cout << square(4) << std::endl;

Writing executable to square.x

Could not link executable

!./square.x

sh: 1: ./square.x: not found

%%file¶

This magic command copies the content of the cell in a file named filename. There’s an optional -a argument to append the content to the file.

%%file [-a] filename

Example:

%%file tmp.txt
Demo of magic command

Writing tmp.txt

%%file -a tmp.txt
Appending to tmp.txt

Appending to tmp.txt

!cat tmp.txt

Demo of magic command
Appending to tmp.txt

%timeit¶

Measures the execution time for a line statement (%timeit) or for a block of statements (%%timeit).

Usage in line mode:

%timeit [-n<N> -r<R> -p<P>] statement

Usage in cell mode:

%%timeit [-n<N> -r<R> -p<P>]
statements

Example:

#include <xtensor/xtensor.hpp>

auto x = xt::linspace<double>(1, 10, 100);

%timeit xt::eval(xt::sin(x));

22.4 us +- 190 ns per loop (mean +- std. dev. of 7 runs 10000 loops each)

%timeit -n 10 -r 1 -p 6 xt::eval(xt::sin(x));

23.3356 us +- 0 ns per loop (mean +- std. dev. of 1 run, 10 loops each)

%%timeit 
auto y = xt::linspace<double>(1, 10, 100);
xt::eval(xt::sin(y) * xt::cos(x));

37.4 us +- 707 ns per loop (mean +- std. dev. of 7 runs 10000 loops each)

Optional arguments: