For example, a library might want to specialize std::swap , std::hash or std::less for its own type. You can overload std::swap instead, but you can't do that for std::hash and std::less. The way to do this is close your library-specific namespace, open namespace std , and put the specialization there. Except if the header of the library is included in arbitrarily deeply nested namespaces, it cannot close those namespaces.
The namespace std it attempts to open won't be ::std , but ::YourStuff::std , which probably doesn't contain any primary template to specialize, and even if it did, that would still be the wrong thing to do.
Finally, things in a namespace simply have different names than things outside. If your library isn't header-only but has a compiled part, the compiled part probably didn't nest everything in the namespace, so the stuff in the library has different names than the stuff you just included. In other words, your program will fail to link. So in theory, you can design headers that work when included within a namespace, but they're annoying to use have to bubble up all dependencies to the includer and very restricted can't use fully qualified names or specialize stuff in another library's namespace, must be header-only.
So don't do it. But you have an old library that doesn't use namespaces, and you want to update it to use them without breaking all your old code. Here's what you should do:.
First, you add a subdirectory to your library's include directory. Call it "namespaced" or something like that. Next, move all the headers into that directory and wrap their contents in a namespace. Then you add forwarding headers to the base directory. For each file in the library, you add a forwarder that looks like this:. Dispose regardless of how control leaves the block if the object is not null at that point.
Dispose " in the finally block, but in a format that expresses the intent. It exists so that if control leaves the block in any way an exception occurs or a return or break is encountered , the. Dispose is still called. Asked 5 Months ago Answers: 5 Viewed 42 times. Now those were the difference.
Names are within namespace std No name-clashes anymore. I'd suggest using unix util xxd for this. Here's just a few things that can go wrong if you include a header from within a namespace. Here's what you should do: First, you add a subdirectory to your library's include directory. Jeremy J Starcher. They differ in a couple of ways: using creates a new scope.
Kaizen Tamashi. However, if backslashes occur within file, they are considered ordinary text characters, not escape characters. None of the character escape sequences appropriate to string constants in C are processed. The exact behavior of the preprocessor varies between compilers.
The following answer applies for GCC and several other compilers. You can use the -I flag for GCC to tell it that, when it encounters an include with angled brackets, it should also search for headers in the directory after -I.
GCC will treat the directory after the flag as if it were the includes directory. For instance, if you have a file called myheader. Without the -I flag, you will have to use include "myheader. The "file" include tells the preprocessor to search the source file's directory first , and then revert to -I and predefined. All destinations are searched anyway, only the order of search is different. The rest is implementation-defined. Note that the standard does not tell any relation between the implementation-defined manners.
The first form searches in one implementation-defined way, and the other in a possibly other implementation-defined way. Formally you'd have to read the manual for your compiler, however normally by tradition the include " So if you want to generate dependency rules using the GCC -M option for exemple , you must use the quoted form for the files that should be included in the dependency tree.
Like many programmers, I have used the informal convention of using the "myApp. However, the C standard states that the search order is implementation specific, which can make portability complicated. To make matters worse, we use jam, which automagically figures out where the include files are. You can use relative or absolute paths for your include files.
I don't know when it changed. Just use forward slashes for compatibility with 'nix Windows will accept that. If you are really worried about it, use ". Here's the MSDN explanation copied here for your convenience. For include "" a compiler normally searches the folder of the file which contains that include and then the other folders.
An include with angle brackets will search an "implementation-dependent list of places" which is a very complicated way of saying "system headers" for the file to be included.
An include with quotes will just search for a file and, "in an implementation-dependent manner", bleh. The gcc documentation has a compiler specific description which although being specific to gcc and not the standard, is a lot easier to understand than the attorney-style talk of the ISO standards. Many of the answers here focus on the paths the compiler will search in order to find the file.
This is not purely hypothetical. There is at least one compiler that work that way. If the header file is predefined then you would simply write the header file name in angular brackets, and it would look like this assuming we have a predefined header file name iostream :. If you the programmer wrote your own header file then you would write the header file name in quotes.
So, suppose you wrote a header file called myfile. In the class implementation for example, Seller. So the compiler will check in the locations where standard library headers are residing. So the compiler will check for these header files in the current folder or -I defined folders. The first one is include which tells the preprocessor to look for the file in the predefined default location. And the second type is include "filename" which tells the preprocessor to look for the file in the current directory first, then look for it in the predefined locations user have set up.
For your own files that needs to be included in another program you have to use the include "filename" syntax. First, looks for the presence of header file in the current directory from where directive is invoked. If not found, then it searches in the preconfigured list of standard system directories. This looks for the presence of header file in the current directory from where directive is invoked. The exact search directory list depends on the target system, how GCC is configured, and where it is installed.
You can find the search directory list of your GCC compiler by running it with -v option. You can add additional directories to the search path by using - I dir , which causes dir to be searched after the current directory for the quote form of the directive and ahead of the standard system directories. Basically, the form "xxx" is nothing but search in current directory; if not found falling back the form. The simple general rule is to use angled brackets to include header files that come with the compiler.
Use double quotes to include any other header files. Most compilers do it this way. If you are a novice programmer, that page should help you understand all that. I learned it from here, and I have been following it at work. These libraries can be stdio. For more information about preprocessors and header. Read C - Preprocessors. To see the search order on your system using gcc, based on current configuration , you can execute the following command.
You can find more detail on this command here. Apple LLVM version In general the difference is where the preprocessor searches for the header file:. Both include are used to add or include header file in the program, but first is to include system header files and later one for user defined header files. Check the gcc docs gcc include files.
Then search the default include path. You can use command like this to print the default include path:. Includes a file in the current directory in which it was compiled. Double quotes can specify a full file path to a different location as well. The implementation-defined warnings generated by the compiler can and will treat system libraries differently than program libraries.
Stack Overflow for Teams — Collaborate and share knowledge with a private group. Create a free Team What is Teams? Collectives on Stack Overflow. Learn more. Ask Question. Asked 13 years, 2 months ago. Active 1 month ago. Viewed k times.
Improve this question. Anthon For the behavior of Visual Studio, please check: docs. Add a comment. Active Oldest Votes. Improve this answer. Caleb k 19 19 gold badges silver badges bronze badges. The statement: "the preprocessor searches in the same directory See answer from piCookie.
While your answer may appear to be "true", because this is how many implementations work by convention, you should take a close look at aib's and piCookie's answers. They both point out backed by the wording of the C standard that the real distinction is inclusion of a "header" versus inclusion of a "source file" and no, this doesn't mean ". A header does not necessarily need to be a file a compiler could e.
I was interested in this question because I was curious what the actual answer is, but I know this is not true because at least with gcc when you specify an additional include path with -I that will search for files specified with include "filename.
Those who don't like the answer, please, give one practical example, where it is wrong. Standards exist for a reason. Show 3 more comments. The only way to know is to read your implementation's documentation. A preprocessing directive of the form include "q-char-sequence" new-line causes the replacement of that directive by the entire contents of the source file identified by the specified sequence between the " delimiters.
A preprocessing directive of the form include pp-tokens new-line that does not match one of the two previous forms is permitted. Fritz 13 13 silver badges 27 27 bronze badges. So what is the difference? Your implementation may do that, and mine may not. As with all of these situations, concrete examples especially of common scenarios are greatly useful and equally appreciated. Needlessly obtuse generic answers don't have as much practical use.
Show 13 more comments. AKS 4, 2 2 gold badges 27 27 silver badges 48 48 bronze badges. According to the standard which piCookie quotes from in his answer , the only real difference is "header" versus "source file". The search mechanism is implementation-defined either way. Using double quotes means that you intend to include a "source file", while angle brackets mean you intend to include a "header" which, as you say, may not be a file at all. See Dan Moulding's comment to quest49's answer; standard headers don't have to be in file form, they can be built-in.
I've been reading this "standard headers don't have to be in file form" for a decade. Care to provide a real-world example? Maxim Yegorushkin: I can't think of any existing real-world examples either; however, no complete C11 compiler can exist for MS-DOS unless headers don't have to be files.
This is because some of the C11 header names are not compatible with the "8. Show 4 more comments. According to The Open Group Base Specifications Issue 7 , -I directory Change the algorithm for searching for headers whose names are not absolute pathnames to look in the directory named by the directory pathname before looking in the usual places.
Yann Droneaud Yann Droneaud 4, 1 1 gold badge 20 20 silver badges 39 39 bronze badges. What is exact source of the text? This was my first thought too. The manpage for gcc includes this as do others. There's also a similar thing for libraries - -L. It is an error if there is anything other than comments on the line after the file name. Peter Mortensen 29k 21 21 gold badges 97 97 silver badges bronze badges. Suraj Jain Suraj Jain 4, 25 25 silver badges 39 39 bronze badges.
What is a "quote directory"? What does "system" mean in the phrase "system header file"? I find that computer scientists throw around this word "system" a lot and I often can't tell if it means "operating system", "computer system", or something else. This is the best answer. That way it's clear in code whether a header is a system header. I agree this is the best answer.
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