For identifiers formed from multiple words, separate each component with underscore rather than capitalization. Thus, use my_var instead of myVar or MyVar.
Avoid capitalization except for class names; class names should begin with a capital letter. This exception from Google coding guidelines exists because the server has a history of using My_class. It will be confusing to mix the two (from a code-review perspective).

class Item;
class Query_arena;
class Log_event;

Avoid function names, structure elements, or variables that begin or end with '_'.
Use long function and variable names in English. This makes your code easier to read for all developers.
We used to have the rule: "Structure types are typedef'ed to an all-upper-case identifier." That rule has been deprecated for C++ code. Do not add typedefs for structs/classes in C++.
All #define declarations should be in upper case.

#define MY_CONSTANT 15

Enumeration names should begin with enum_.
Function declarations (forward declarations) have parameter names in addition to parameter types.