C++ default constructor: string params vs string params()

Question

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Is no parentheses on a constructor with no arguments a language standard?

Can anyone explain why these line don't give me an error:

string params;
params+="d";

but these lines:

string params();
params+="d";

give me this error: error C2659: '+=' : function as left operand

See [Most vexing parse](http://en.wikipedia.org/wiki/Most_vexing_parse). — Some programmer dude, Oct 16 '12 at 09:05

score 7 · Accepted Answer · answered Oct 16 '12 at 09:03

7

This is not object:

 string params();

This is function returning string:

 string params();

Like this:

 string params(void);

So the error now is obvious: function as left operand

answered Oct 16 '12 at 09:03

PiotrNycz

so string params() is not an object but string params("word") is an object? – kakush Oct 16 '12 at 09:07
@kakush Yes - "word" is not type - it is value - so it is object. When compiler is in doubt if the expression is function declaration or object - it always select function declaration. In your example with "word" - there is no doubt. – PiotrNycz Oct 16 '12 at 09:09
thanks @PiotrNycz . so if I want to use string's empty const' how do i do that? – kakush Oct 16 '12 at 09:12
1

@kakush: you write `string params;`, which calls the class's default constructor if it has one declared (`string` does). If you want some type `T` that might not have a constructor at all (for instance built-in types), you can write `T t = T();` provided that it's copyable (or movable in C++11). If you have a type that *either* has a declared default constructor *or* is copyable, but you don't know which, then in C++03 you have a problem. In C++11 you can write `T t{};` to value-initialize anything. – Steve Jessop Oct 16 '12 at 09:15

score 1 · Answer 2 · answered Oct 16 '12 at 09:08

In the first case with the

string params;

creates a string instance using a default constructor.

In the second case the

string params();

creates a pointer to a function returning string. On that type the operator+=(const string&) is apparently not defined.

Yes, it is a bit confusing feature of the language because when you use not default constructor, you could really write e.g.

string params("d");

2 Answers2