C++ default constructor: string params vs string params() [duplicate]

问题

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Closed 7 years ago.

Possible Duplicate:
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

回答1:

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

This problem has own name: http://en.wikipedia.org/wiki/Most_vexing_parse

回答2:

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");

来源：https://stackoverflow.com/questions/12911078/c-default-constructor-string-params-vs-string-params