{"id":3949,"date":"2013-12-05T15:15:36","date_gmt":"2013-12-05T13:15:36","guid":{"rendered":"http:\/\/www.cloudcomp.ch\/?p=3949"},"modified":"2014-03-28T15:22:08","modified_gmt":"2014-03-28T13:22:08","slug":"const-in-cpp-a-brief-overview","status":"publish","type":"post","link":"https:\/\/blog.zhaw.ch\/icclab\/const-in-cpp-a-brief-overview\/","title":{"rendered":"Const in C++, a brief overview"},"content":{"rendered":"

A word of warning at the beginning: This post is about C++ and not about C! So whatever you read here may not necessarily apply to C. Nevertheless you may have a look Sugih’s page which features are in C<\/a>. Besides I’m not going to explain what references and pointers are.<\/p>\n

Introduction<\/h1>\n

While working on the KIARA project<\/a> writing C++11 code I was faced with the task of passing variables to functions which will not alter these. Or pure getter methods. And after reading a post about C++ constness<\/a> I was left with more questions than really understanding. A quick search revealed thorough FAQ about const usage in C++<\/a>. Here I’d like to make a short round-up of it.<\/p>\n

How to read const<\/code><\/h1>\n

First of all, read it from right-to-left. For instance int const* const p<\/code> would be read as “p is a constant pointer to a constant int”.<\/p>\n

A few examples<\/h1>\n

string const& s<\/code><\/h2>\n

Let’s say you read something like this:<\/p>\n

\nvoid MyClass::func (std::string const& s);\n<\/pre>\n
So reading from right-to-left this would say “s is reference to a constant std::string”. This implies that func<\/code> is not going to modify s<\/code>. But keep in mind that you may have a dangling reference here, especially if you work with multi-threaded code. If this would be a pointer instead of a reference it is still possible the pointer to be NULL.<\/p>\n
string* const s<\/code><\/h2>\n
\nvoid MyClass::func (std::string* const s);\n<\/pre>\n
Again, read from right-to-left: “s is a constant pointer to a string”. s<\/code> may be modified (the object itself) but s<\/code> may not point to a different object. As you may have already noticed it’s not that hard to read when you stick to the “read from right-to-left” rule.<\/p>\n
string const* const s<\/code><\/h2>\n
\nvoid MyClass::func (std::string const* const s);\n<\/pre>\n
This would read “s is a constant pointer to a constant string” and therefore the function guarantees (respectively the compiler enforces it) that the content of s<\/code> may not be changed nor may s<\/code> point to a different object.<\/p>\n
Similar consts<\/h2>\n
Obviously some signatures mean the same thing:<\/p>\n
\nvoid MyClass::func (const std::string& s); \/\/ you may know this from C\nvoid MyClass::func (std::string const& s);\n\nvoid MyClass::func (const std::string* s);\nvoid MyClass::func (std::string const* s); \/\/ equivalent\nvoid MyClass::func (std::string* const s); \/\/ Pitfall: NOT equivalent\n<\/pre>\n
Now, it doesn’t matter which version you use but is more a decision you have to make. If previously written code opts for one variant you should for the sake of consistency use the same way.<\/p>\n
Methods that do not change its object<\/h2>\n
When you write a pure getter method you may want to tell your compiler that the following code may not change the object itself.<\/p>\n
\nstd::string const& MyClass::func () const;\n<\/pre>\n
The last const indicates that this function may not alter its objects data.<\/p>\n
Conclusion<\/h1>\n
C++ is not a language you master in a few hours of training, C++ is like meditating and takes daily training.<\/p>\n
Schlagw\u00f6rter: C++<\/a>, const<\/a>, pointer<\/a>, reference<\/a>
<\/div>","protected":false},"excerpt":{"rendered":"
A word of warning at the beginning: This post is about C++ and not about C! So whatever you read here may not necessarily apply to C. Nevertheless you may have a look Sugih’s page which features are in C. Besides I’m not going to explain what references and pointers are. Introduction While working on […]<\/p>\n","protected":false},"author":61,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"ngg_post_thumbnail":0,"footnotes":""},"categories":[15],"tags":[60,96,258,280],"features":[],"class_list":["post-3949","post","type-post","status-publish","format-standard","hentry","category-howtos","tag-c","tag-const","tag-pointer","tag-reference"],"yoast_head":"\nConst in C++, a brief overview - Service Engineering (ICCLab & SPLab)<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/blog.zhaw.ch\/icclab\/const-in-cpp-a-brief-overview\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Const in C++, a brief overview\" \/>\n<meta property=\"og:description\" content=\"A word of warning at the beginning: This post is about C++ and not about C! 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