Style Guide/Best Practice C Cpp

Best Practice

Note that this page is in addition to the style guide which covers stylistic aspects of writing code, this page covers other topics relating to writing code for Blender.

Order of Operations for "sizeof(..)"

When calculating the size of a value in bytes, order sizeof first, eg: sizeof(type) * length, this avoids integer overflow errors by promoting the second value to size_t in the case it's a smaller type such as an int.

Note that for array allocation, we have MEM_malloc_arrayN and MEM_calloc_arrayN.

Comment unused arguments in C++ code

When a C++ function has an unused argument, prefer declaring it like int /*my_unused_var*/ to using the UNUSED() macro. This is because the macro does not work properly for MSVC-- it only prevents using the variable but does not suppress the warning. It also has a more complex implementation.

Avoid Unsafe String C-API's

Given the current state of Blender's internal data structures (DNA structs's for example), fixed size char buffers are used making it necessary to use low level string manipulation.

Unsafe C-API functions should be avoided as these have been a cause of bugs historically (see examples).

This table lists function to avoid and alternatives that should be used instead.

Unsafe C-API	Safe Alternative
strcpy, strncpy	BLI_strncpy, or STRNCPY macro.
sprintf, snprintf	BLI_snprintf, BLI_snprintf_rlen or SNPRINTF, SNPRINTF_RLEN macros.
vsnprintf, vsprintf	BLI_vsnprintf, or VSNPRINTF, VSNPRINTF_RLEN macros.
strcat, strncat	BLI_strncat. BLI_string_join may also be an alternative for concatenating strings.

Notes relating to fixed size char buffer use:

• Unless otherwise stated fixed sized char buffers must be null terminated.
• Queries that rely on null termination are acceptable such as strlen, strstr and related functions.
• When constructing UTF8 encoded strings which may not fit destination buffer, copy the strings with BLI_strncpy_utf8, or use BLI_str_utf8_invalid_strip on the resulting string.
• When performing low level operations on byte arrays, it's preferable to calculate sizes and use memcpy to construct the buffer.
• Complex logic to construct fixed size char buffers should assert the final size fits withing buffer size.

Exceptions

• Libraries in extern/ are not maintained as part of Blender's code, this policy doesn't apply there.
• StringPropertyRNA::get callbacks (defined in rna_*.cc) uses strcpy as the string must be large enough to hold a string length defined by StringPropertyRNA::length.