9 Code Formatting

Conditional statements and loops

9.1 Curly braces should be used in all conditionals, loops, etc. even when the content inside the braces is a “one-liner”.

This helps prevent coding errors and misinterpretation of intent. For example, this:

if (done) { ... }

is preferable to this:

if (done) ...

9.2 One-liners may be used for “if” conditionals with “else/else if” clauses when the resulting code is clear.

For example, either of the following styles may be used:

if (done) {
   id = 3;
} else {
   id = 0;


if (done) { id = 3; } else { id = 0; }

9.3 Complex “if/else if” conditionals with many “else if” clauses should be avoided.

Such statements can always be refactored using local boolean variables or “switch” statements. Doing so often makes code easier to read and understand and may improve performance.

9.4 An explicit test for zero/nonzero must be used in a conditional unless the tested quantity is a boolean or pointer type.

For example, a conditional based on an integer value should use:

if (num_lines != 0) { ... }


if (num_lines) { ... }

White space and code alignment

Most conventions for indentation, spacing and code alignment preferred by the team are enforced by using the clang-format tool.

There are several build system targets related to code formatting grouped under the check and style targets. The former verify that the code is properly formatted, while the latter modify source files to conform to axom’s rules.


Axom’s style targets modify source files. Please ensure that you’ve committed/staged all your changes before running them.


When axom is configured to use the make-based generator, the entire codebase can be formatted by running make clangformat_style from the build directory. Simiarly, one can verify that the code if properly formatted by running make clangformat_check. There are also component-specific variants for these targets, e.g. for axom’s core component, we have core_clangformat_style and core_clangformat_check.

Not all preferred formatting conventions are supported by clang-format. The following guidelines provide additional recommendations to make code easier to read and understand.

White space enhances code readability

9.5 Blank lines and indentation should be used throughout code to enhance readability.

Examples of helpful white space include:

  • Between operands and operators in arithmetic expressions.
  • After reserved words, such as “while”, “for”, “if”, “switch”, etc. and before the parenthesis or curly brace that follows.
  • After commas separating arguments in functions.
  • After semi-colons in for-loop expressions.
  • Before and after curly braces in almost all cases.

9.6 White space must not appear between a function name and the opening parenthesis to the argument list. In particular, if a function call is broken across source lines, the break must not come between the function name and the opening parenthesis.

9.7 Tabs must not be used for indentation since this can be problematic for developers with different text editor settings.

Vertical alignment helps to show scope

9.8 When function arguments (in either a declaration or implementation) appear on multiple lines, the arguments should be vertically aligned for readability.

9.9 All statements within a function body should be indented within the surrounding curly braces.

9.10 All source lines in the same scope should be vertically aligned. Continuation of previous lines may be indented if it make the code easier to read.

Break lines where it makes sense

9.11 When a line is broken at a comma or semi-colon, it should be broken after the comma or semi-colon, not before. This helps make it clear that the statement continues on the next line.

9.12 When a source line is broken at an arithmetic operator (i.e., +, -, etc.), it should be broken after the operator, not before.

Use parentheses for clarity

9.13 Parentheses should be used in non-trivial mathematical and logical expressions to clearly indicate structure and intended order of operations. Do not assume everyone who reads the code knows all the rules for operator precedence.