Question

I am a C++ beginner, so sorry if the question is too basic.

I have tried to collect the string constrcturs and try all them out (to remember them).

string strA();          // string(); empty string // incorrect
string strB("Hello");   // string( const char* str)
string strC("Hello",3); // string( const char* str, size_type length)
string strD(2,'c');     // string( size_type lenght, const char &c)
string strE(strB);      // string( const string& s)

cout << strA << endl;
cout << strB << endl;
cout << strC << endl;
cout << strD << endl;
cout << strE << endl;

All of them works except for the strA. It prints “1”. Why? Whats the type of the strA in this case? How can I check the type of stuff when I am unsure?

I have noticed that the correct way is this (which by the way seems to be inconsistent with the other constructors, sometimes parens sometimes no parens):

string strA;

ps: question in bold, usual irrelevant answers will be downvoted.

Answer 1

This is a very popular gotcha. C++ grammar is ambiguous. One of the rules to resolve ambiguities is “if something looks like declaration it is a declaration”. In this case instead of defining a variable you declared a function prototype.

string strA();

is equivalent to

string strA(void);

a prototype of a no-arg function which returns string.

If you wish to explicitly call no-arg constructor try this:

string strA=string();

It isn’t fully equivalent – it means ‘create a temporary string using no-arg constructor and then copy it to initialize variable strA’, but the compiler is allowed to optimize it and omit copying.

EDIT:
Here is an appropriate item in C++ FAQ Lite