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Inja
3.1.0
A Template Engine for Modern C++
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Inja is a template engine for modern C++, loosely inspired by jinja for python. It has an easy and yet powerful template syntax with all variables, loops, conditions, includes, callbacks, and comments you need, nested and combined as you like. Inja uses the wonderful json library by nlohmann for data input. Most importantly, inja needs only two header files, which is (nearly) as trivial as integration in C++ can get. Of course, everything is tested on all relevant compilers. Here is what it looks like:
Inja is a headers only library, which can be downloaded from the releases or directly from the include/
or single_include/
folder. Inja uses nlohmann/json.hpp
(>= v3.8.0) as its single dependency, so make sure it can be included from inja.hpp
. json can be downloaded here. Then integration is as easy as:
If you are using the Meson Build System, then you can wrap this repository as a subproject.
If you are using Conan to manage your dependencies, have a look at this repository. Please file issues here if you experience problems with the packages.
You can also integrate inja in your project using Hunter, a package manager for C++.
If you are using vcpkg on your project for external dependencies, then you can use the inja package. Please see the vcpkg project for any issues regarding the packaging.
If you are using cget, you can install the latest development version with cget install pantor/inja
. A specific version can be installed with cget install pantor/inja@v2.1.0
.
On macOS, you can install inja via Homebrew and brew install inja
.
This tutorial will give you an idea how to use inja. It will explain the most important concepts and give practical advices using examples and executable code. Beside this tutorial, you may check out the documentation.
The basic template rendering takes a template as a std::string
and a json
object for all data. It returns the rendered template as an std::string
.
For more advanced usage, an environment is recommended.
The environment class can be configured to your needs.
Variables are rendered within the {{ ... }}
expressions.
If no variable is found, valid JSON is printed directly, otherwise an inja::RenderError
is thrown.
Statements can be written either with the {% ... %}
syntax or the ##
syntax for entire lines. Note that ##
needs to start the line without indentation. The most important statements are loops, conditions and file includes. All statements can be nested.
In a loop, the special variables loop/index (number)
, loop/index1 (number)
, loop/is_first (boolean)
and loop/is_last (boolean)
are defined. In nested loops, the parent loop variables are available e.g. via loop/parent/index
. You can also iterate over objects like {% for key, value in time %}
.
Conditions support the typical if, else if and else statements. Following conditions are for example possible:
You can either include other in-memory templates or from the file system.
Inja will throw an inja::RenderError
if an included file is not found. To disable this error, you can call env.set_throw_at_missing_includes(false)
.
Variables can also be defined within the template using the set statment.
A few functions are implemented within the inja template syntax. They can be called with
In the default configuration, no whitespace is removed while rendering the file. To support a more readable template style, you can configure the environment to control whitespaces before and after a statement automatically. While enabling set_trim_blocks
removes the first newline after a statement, set_lstrip_blocks
strips tabs and spaces from the beginning of a line to the start of a block.
With both trim_blocks
and lstrip_blocks
enabled, you can put statements on their own lines. Furthermore, you can also strip whitespaces for both statements and expressions by hand. If you add a minus sign (-
) to the start or end, the whitespaces before or after that block will be removed:
Stripping behind a statement or expression also removes any newlines.
You can create your own and more complex functions with callbacks. These are implemented with std::function
, so you can for example use C++ lambdas. Inja Arguments
are a vector of json pointers.
You can also add a void callback without return variable, e.g. for debugging:
Comments can be written with the {# ... #}
syntax.
Inja uses exceptions to handle ill-formed template input. However, exceptions can be switched off with either using the compiler flag -fno-exceptions
or by defining the symbol INJA_NOEXCEPTION
. In this case, exceptions are replaced by abort()
calls.
Inja uses string_view
from C++17, but includes the polyfill from martinmoene. This way, the minimum version is C++11. Currently, the following compilers are tested:
The unit tests fail to compile with GCC 4.8 but should just work fine. A complete list of supported compiler / os versions can be found in the CI definition.