# sol2 3.5.0 documentation

## config and safety

### Search the Documentation

# config and safety

sol was designed to be correct and fast, and in the pursuit of both uses the regular `lua_to{x}` functions of Lua rather than the checking versions (`lua_check{X}`) functions. The API defaults to paranoidly-safe alternatives if you have a `#define SOL_ALL_SAFETIES_ON` before you include sol, or if you pass the `SOL_ALL_SAFETIES_ON` define on the build command for your build system. By default, it is off and remains off unless you define this, even in debug mode.

## config

Note that you can obtain safety with regards to functions you bind by using the protect wrapper around function/variable bindings you set into Lua. Additionally, you can have basic boolean checks when using the API by just converting to a sol::optional<T> when necessary for getting things out of Lua and for function arguments.

Also note that you can have your own states use sol2’s safety panics and similar to protect your code from crashes. See sol::state automatic handlers for more details.

### Safety Config

`SOL_SAFE_USERTYPE` triggers the following change:

  - If the userdata to a usertype function is nil, will trigger an error instead of letting things go through and letting the system segfault/crash

  - Turned on by default with clang++, g++ and VC++ if a basic check for building in debug mode is detected (lack of `_NDEBUG` or similar compiler-specific checks)

`SOL_SAFE_REFERENCES` triggers the following changes:

  - Checks the Lua type to ensure it matches what you expect it to be upon using sol::reference derived types, such as `sol::thread`, `sol::function`, etc…

  - Turned on by default with clang++, g++ and VC++ if a basic check for building in debug mode is detected (lack of `_NDEBUG` or similar compiler-specific checks)

`SOL_SAFE_FUNCTION_CALLS` triggers the following changes:

  - `sol::stack::call` and its variants will, if no templated boolean is specified, check all of the arguments for a function call

  - All calls from Lua will have their arguments checked

  - Turned on by default with clang++, g++ and VC++ if a basic check for building in debug mode is detected (lack of `_NDEBUG` or similar compiler-specific checks)

`SOL_SAFE_FUNCTIONS` triggers the following change:
  - All uses of `sol::function` and `sol::stack_function` will default to `sol::protected_function` and `sol::stack_protected_function`, respectively, rather than `sol::unsafe_function` and `sol::stack_unsafe_function`

      - Note this does not apply to `sol::stack_aligned_function`: this variant must always be unprotected due to stack positioning requirements, especially in use with `sol::stack_count`

  - Will make any `sol::state_view::script` calls default to their safe variants if there is no supplied environment or error handler function

  - Not turned on by default under any detectible compiler settings: this MUST be turned on manually

`SOL_SAFE_NUMERICS` triggers the following changes:

  - Numbers will also be checked to see if they fit within a `lua_Number` if there is no `lua_Integer` type available that can fit your signed or unsigned number

  - You can opt-out of this behavior with `SOL_NO_CHECK_NUMBER_PRECISION`

  - This option is required to differentiate between floats/ints in overloads

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_SAFE_GETTER` triggers the following changes:

  - `sol::stack::get` (used everywhere) defaults to using `sol::stack::check_get` and dereferencing the argument. It uses `sol::type_panic` as the handler if something goes wrong

  - Affects nearly the entire library for safety (with some blind spots covered by the other definitions)

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_ALL_SAFETIES_ON` triggers the following changes:

  - If `SOL_SAFE_USERTYPE`, `SOL_SAFE_REFERENCES`, `SOL_SAFE_FUNCTION`, `SOL_SAFE_NUMERICS`, `SOL_SAFE_GETTER`, and `SOL_SAFE_FUNCTION_CALLS` are not defined, they get defined and the effects described above kick in

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_DEFAULT_PASS_ON_ERROR` triggers the following changes:

  - The default error handler for `sol::state_view::script_` functions is `sol::script_pass_on_error` rather than `sol::script_throw_on_error`

  - Passes errors on through: very dangerous as you can ignore or never be warned about errors if you don’t catch the return value of specific functions

  - Not turned on by default: this MUST be turned on manually

  - Don’t turn this on unless you have an extremely good reason

  - DON’T TURN THIS ON UNLESS YOU HAVE AN EXTREMELY GOOD REASON

`SOL_NO_CHECK_NUMBER_PRECISION` triggers the following changes:

  - If `SOL_SAFE_NUMERICS` is defined, turns off number precision and integer precision fitting when pushing numbers into sol2

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_FUNCTION_CALL_VALUE_SEMANTICS` triggers the following changes:

  - Function arguments and returns into Lua (`sol:function` and similar) do not pass their arguments by reference: they get copied

  - Individual types can be blessed by template specializing `sol::is_value_semantic_for_function<T>`

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_STRINGS_ARE_NUMBERS` triggers the following changes:
  - Allows automatic to-string conversions for numbers

      - `lua_tolstring` conversions are not permitted on numbers through sol2 by default: only actual strings are allowed

      - This is necessary to allow sol::overload to work properly

  - `sol::stack::get` and `sol::stack::check_get` will allow anything that Lua thinks is number-worthy to be number-worthy

  - This includes: integers, floating-point numbers, and strings

  - This does not include: booleans, types with `__tostring` enabled, and everything else

  - Overrides safety and always applies if it is turned on

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_CHECK_ARGUMENTS` is a deprecated define that will be removed soon:

  - It is simply an alias for `SOL_ALL_SAFETIES_ON`

  - Please change all your code to use `SOL_ALL_SAFETIES_ON`

### Feature Config

`SOL_USE_BOOST` triggers the following change:

  - Attempts to use `boost::optional` instead of sol’s own `optional`

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_PRINT_ERRORS` triggers the following change:

  - Includes `<iostream>` and prints all exceptions and errors to `std::cerr`, for you to see

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_GET_FUNCTION_POINTER_UNSAFE` triggers the following change:

  - Allows function pointers serialized into Lua as a callable to be retrieved back from Lua in a semi-proper manner

  - This is under NO circumstances type safe

      - It WILL break `sol::overload` type checking and will not discriminate properly between function types

      - It WILL happily let you retrieve an `int(*)(int, int int)` from a `void(*)()` function pointer, and shatter your runtime if you call it

  - This is an advanced, experimental feature for experts only and requires the user has perfect type safety in both C++ and Lua

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_CONTAINERS_START` triggers the following change:

  - If defined and is an integral value, is used to adjust the container start value

  - Applies to C++ containers only (not Lua tables or algorithms)

  - Defaults to 1 (containers in Lua count from 1)

`SOL_USE_INTEROP` triggers the following change:
  - Allows the use of `extensible<T>` to be used with `userdata_checker` and `userdata_getter` to retrieve non-sol usertypes

      - Particularly enables non-sol usertypes to be used in overloads

      - See the stack documentation for details

  - May come with a slight performance penalty: only recommended for those stuck with non-sol libraries that still need to leverage some of sol’s power

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_DEFAULT_AUTOMAGICAL_USERTYPES` triggers the following change:

  - Either turns on (`!= 0`) or turns off (`== 0`) automagical usertype detection by default.

  - Automagical usertypes search for specific C++ conventions to define common methods for the end-user.

  - Some automagical methods might cause huge compiler errors, and some people have code bases with different conventions.

  - Turned on by default. This must be turned off manually.

`SOL_NO_THREAD_LOCAL` triggers the following change:

  - If this is turned on, simply removes all usages of the `thread_local` keyword in sol2.

  - This is useful for lower versions of iOS and Android, which do not have threading capabilities at all and so the use of the keyword provides no additional guarantees.

  - Not turned on by default under any settings: this MUST be turned on manually

`SOL_ID_SIZE` triggers the following change:

  - If this is defined to a numeric value, it uses that numeric value for the number of bytes of input to be put into the error message blurb in standard tracebacks and `chunkname` descriptions for `.script`/`.script_file` usage.

  - Defaults to the `LUA_ID_SIZE` macro if defined, or some basic internal value like 2048.

`SOL_LUAJIT` triggers the following change:

  - Has sol2 expect LuaJIT, and all of its quirks.

  - Turns on by default if the macro `LUAJIT_VERSION` is detected from including Lua headers without any work on your part. Can also be manually defined.

### Memory Config

`SOL_NO_MEMORY_ALIGNMENT` triggers the following changes:

  - Memory is no longer aligned and is instead directly sized and allocated

  - If you need to access underlying userdata memory from sol, please see the usertype memory documentation

  - Not turned on by default under any settings: this MUST be turned on manually

### Linker Config

`SOL_USING_CXX_LUA` triggers the following changes:

  - Lua includes are no longer wrapped in `extern "C" {}` blocks

  - `SOL_NO_LUA_HPP` takes less precedence if this is explicitly turned on

  - Turns on `SOL_EXCEPTIONS_SAFE_PROPAGATION` automatically for you

  - Only use this if you know you’ve built your LuaJIT with the C++-specific invocations of your compiler (Lua by default builds as C code and is not distributed as a C++ library, but a C one with C symbols)

`SOL_USING_CXX_LUA_JIT` triggers the following changes:

  - LuaJIT includes are no longer wrapped in `extern "C" {}` blocks

  - Turns on `SOL_EXCEPTIONS_SAFE_PROPAGATION` automatically for you

  - Only use this if you know you’ve built your LuaJIT with the C++-specific invocations of your compiler

  - LuaJIT by default builds as C code, but includes hook to handle C++ code unwinding: this should almost never be necessary for regular builds

`SOL_EXCEPTIONS_ALWAYS_UNSAFE` triggers the following changes:

  - If any of the `SOL_USING_CXX_*` defines are in play, it does NOT automatically turn on `SOL_EXCEPTIONS_SAFE_PROPAGATION` automatically

  - This standardizes some behavior, since throwing exceptions through the C API’s interface can still lead to undefined behavior that Lua cannot handle properly

`SOL_EXCEPTIONS_SAFE_PROPAGATION` triggers the following changes:

  - try/catch will not be used around C-function trampolines when going from Lua to C++

  - try/catch will not be used in `safe_`/`protected_function` internals

  - Should only be used in accordance with compiling vanilla PUC-RIO Lua as C++, using LuaJIT under the proper conditions, or in accordance with your Lua distribution’s documentation

Tests are compiled with this on to ensure everything is going as expected. Remember that if you want these features, you must explicitly turn them on all of them to be sure you are getting them.

## memory

Memory safety can be tricky. Lua is handled by a garbage-collected runtime, meaning object deletion is not cleary defined or deterministic. If you need to keep an object from the Lua Runtime alive, use sol::reference or one of its derived types, such as sol::table, sol::object, or similar. These will pin a reference down to an object controlled in C++, and Lua will not delete an object that you still have a reference to through one of these types. You can then retrieve whatever you need from that Lua slot using object’s `obj.as<T>()` member function or other things, and work on the memory from there.

The usertype memory layout for all Lua-instantiated userdata and for all objects pushed/set into the Lua Runtime is also described here. Things before or after that specified memory slot is implementation-defined and no assumptions are to be made about it.

Please be wary of alignment issues. sol2 aligns memory by default. If you need to access underlying userdata memory from sol, please see the usertype memory documentation

## functions

The vast majority of all users are going to want to work with sol::safe_function/sol::protected_function. This version allows for error checking, prunes results, and responds to the defines listed above by throwing errors if you try to use the result of a function without checking. sol::function/sol::unsafe_function is unsafe. It assumes that its contents run correctly and throw no errors, which can result in crashes that are hard to debug while offering a very tiny performance boost for not checking error codes or catching exceptions.

If you find yourself crashing inside of `sol::function`, try changing it to a `sol::protected_function` and seeing if the error codes and such help you find out what’s going on. You can read more about the API on the page itself. You can also define `SOL_SAFE_FUNCTION` as described above, but be warned that the `protected_function` API is a superset of the regular default `function` API: trying to revert back after defining `SOL_SAFE_FUNCTION` may result in some compiler errors if you use things beyond the basic, shared interface of the two types.

As a side note, binding functions with default parameters does not magically bind multiple versions of the function to be called with the default parameters. You must instead use sol::overload.

Warning

Do NOT save the return type of a unsafe_function_result with `auto`, as in `auto numwoof = woof(20);`, and do NOT store it anywhere unless you are exactly aware of the consequences of messing with the stack. See here for more information.