Introduction - what is the most vexing parse anyway?

The title of this site refers to a phrase coined by Scott Meyers. The most vexing parse is a result of the ambiguity of function declarations and variable definitions in C++, but I also think it’s a fun sequence of words.

You’ve probably run into this while trying to define a variable with a call to a default constructor. That happens pretty frequently, so get in the habit of of doing so correctly: by omitting the empty parentheses, as in string str rather than string str(). This is necessary because the latter is ambiguous:

string str(); // Ok, but ambiguous
str = "Hello World!"; // Error! str is not a string
string str2 = str(); // Ok, str is a function returning a string, defined elsewhere

The errors will look something like these:

error: assignment of function ‘std::string str()’

error: request for member ‘c_str’ in ‘str’, which is of non-class type ‘std::string()’

The C++ standard forces the compiler to treat such expressions as declarations¹. In this scenario, the ambiguity can be resolved by removing the parentheses or by using an empty brace initializer (in C++ 11). There are other ways to arrive at an ambiguous expression, and I encourage you to check out Effective STL² where Scott Meyers talks about some of them.