dcsimg
Login | Register   
LinkedIn
Google+
Twitter
RSS Feed
Download our iPhone app
TODAY'S HEADLINES  |   ARTICLE ARCHIVE  |   FORUMS  |   TIP BANK
Browse DevX
Sign up for e-mail newsletters from DevX

By submitting your information, you agree that devx.com may send you DevX offers via email, phone and text message, as well as email offers about other products and services that DevX believes may be of interest to you. DevX will process your information in accordance with the Quinstreet Privacy Policy.


Tip of the Day
Language: C++
Expertise: Beginner
Jun 30, 2000

WEBINAR:

On-Demand

Building the Right Environment to Support AI, Machine Learning and Deep Learning


Forward Declaration for STL String Class

Question:
How do I do a forward declaration for the STL 'string' class? For example:
// Foo.hpp

class string;

class Foo
{
  string f1();
  void f2(const string& str);
  ...
}
On various compliers (MS, Sun, GNU) this produces error messages to the effect of "invalid class definition" or "conflicting class definition."

Answer:
You can't forward declare std::string because it's not the real name of this class; it's a typedef name that hides the cumbersome syntax of the following template instance:

typedef basic_string , 
allocator > string;
Because the compiler must know the exact type of a parameter in order to get the functions named correctly managed, you can't use a typedef name in a forward declaration, unless you provide the typedef declaration as well. In other words, it's OK to use a typedef name in a forward declaration as long as the compiler can see the actual type behind the typedef name. The actual type of the typedef std::string appears in , so you need to #include it in the source file that contains Foo's declaration.

Seemingly, you can place the typedef declaration given above instead of #including the header file. The problem is that this typedef uses other class templates, such as char_traits<> and allocator<>, whose declarations also need to be accessible. Therefore, at the end of the day, the only solution is to #include . Note that many compilers have features for reducing compilation time (e.g., precompiled header and cached headers), so #including the same standard header file in several files shouldn't incur longer compilation time.

DevX Pro
 
Comment and Contribute

 

 

 

 

 


(Maximum characters: 1200). You have 1200 characters left.

 

 

Sitemap
Thanks for your registration, follow us on our social networks to keep up-to-date