Link Error Undefined
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here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us undefined reference to c++ error Learn more about Stack Overflow the company Business Learn more about hiring developers undefined reference to function c++ or posting ads with us Stack Overflow Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack c++ undefined reference to class Overflow Community Stack Overflow is a community of 6.2 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up What is an undefined reference/unresolved external symbol c++ undefined reference to constructor error and how do I fix it? up vote 712 down vote favorite 317 What are undefined reference/unresolved external symbol errors? What are common causes and how to fix/prevent them? Feel free to edit/add your own. c++ linker-error undefined-reference c++-faq unresolved-external share|improve this question edited Aug 4 '14 at 13:33 Arno Duvenhage 805517 asked Sep 24 '12 at 22:27 Luchian Grigore 167k28295455 3 One thing to
Linking Error Undefined Reference To Function
consider adding is how to deal with "undefined vtable" and "undefined typeinfo" errors in particular (since they are less obvious than undefined functions or variables). –Jeremiah Willcock Jul 16 '13 at 18:25 1 I've been marking this question to be a possible dupe of this one. But after going through all of your (brilliant) answers, I can't see this case covered here. I'm aware it's specific about how an IDE sets up the project type and it's linkage dependencies. But that's a such frequently asked question I think it would be worth covered (maybe just with a link to another appropriate dupe) here. If it is already, and I just didn't spot it, forget about this request/comment. –πάντα ῥεῖ Mar 3 '14 at 21:14 @LuchianGrigore 'feel free to add an answer' I preferred to add the relevant link (IMHO) your primary answer, if you'd like to permit. –πάντα ῥεῖ Mar 3 '14 at 22:36 1 Pretty common mistake is that you define a function as a standalone and forget the class selector (e.g. A::) in your .cpp file: You do this (wrong): void myFunc() { /* do stuff */ } Instead of this (right)
error message (or “unresolved external symbol, for Visual C++ users). This is not actually a message from the compiler, but is emitted by the linker, so the first thing to do is to understand what the linker is,
Undefined Reference To Static Variable C++
and what it does. Linker 101 To understand the linker, you have to understand how linker error undefined reference to C++ programs are built. For all but the very simplest programs, the program is composed of multiple C++ source files (also known as undefined reference to c++ linux “translation units”). These are compiled separately, using the C++ compiler, to produce object code files (files with a .o or a .obj extension) which contain machine code. Each object code file knows nothing about the others, so if http://stackoverflow.com/questions/12573816/what-is-an-undefined-reference-unresolved-external-symbol-error-and-how-do-i-fix you call a function from one object file that exists in another, the compiler cannot provide the address of the called function. This is where the the linker comes in. Once all the object files have been produced, the linker looks at them and works out what the final addresses of functions in the executable will be. It then patches up the addresses the compiler could not provide. It does the same for any libraries (.a https://latedev.wordpress.com/2014/04/22/common-c-error-messages-2-unresolved-reference/ and .lib files) you may be using. And finally it writes the executable file out to disk. The linker is normally a separate program from the compiler (for example, the GCC linker is called ld) but will normally be called for you when you use your compiler suite’s driver program (so the GCC driver g++ will call ld for you). Traditionally, linker technology has lagged behind compilers, mostly because it’s generally more fun to build a compiler than to build a linker. And linkers do not necessarily have access to the source code for the object files they are linking. Put together, you get a situation where linker errors, and the reasons for them, can be cryptic in the extreme. Undefined reference Put simply, the “undefined reference” error means you have a reference (nothing to do with the C++ reference type) to a name (function, variable, constant etc.) in your program that the linker cannot find a definition for when it looks through all the object files and libraries that make up your project. There are any number of reasons why it can’t find the definition – we’ll look at the commonest ones now. No Definition Probably the most common reason for unresolved reference errors is that you simply have not defined the thing you are referencing. This code illustrates the problem: int foo(); int main() {
Practice Problems Quizzes Resources Source Code Source Code Snippets C and C++ Tips Finding a Job References Function Reference Syntax Reference Programming FAQ Getting Help Message Board Email About Us Dealing with Compiler Errors http://www.cprogramming.com/tutorial/compiler_linker_errors.html - Surviving the Compilation Process By Alex Allain It's your first C (or C++) program--it's not that long, and you're about to compile it. You hit compile (or enter the build command) and wait. Your compiler spits out fifty lines of text. You pick out words like "warning and "error". Does that mean it worked? you wonder. You look for the resulting executable. Nothing. Damn, you think, I undefined reference guess I have to figure out what this all means... The Types of Compilation Errors First, let's distinguish between the types of errors: most compilers will give three types of compile-time alerts: compiler warnings, compiler errors, and linker errors. Although you don't want to ignore them, compiler warnings aren't something severe enough to actually keep your program from compiling. Usually, compiler warnings are an indication that something might go undefined reference to wrong at runtime. How can the compiler know this at all? You might be making a typical mistake that the compiler knows about. A common example is using the assignment operator ('=') instead of the equality operator ('==') inside an if statement. Your compiler may also warn you about using variables that haven't been initialized and other similar mistakes. Generally, you can set the warning level of your compiler--I like to keep it at its highest level so that my compiler warnings don't turn in to bugs in the running program ('runtime bugs'). Nevertheless, compiler warnings aren't going to stop you from getting your program working (unless you tell your compiler to treat warnings as errors), so they're probably a bit less frustrating than errors. Errors are conditions that prevent the compiler from completing the compilation of your files. Compiler errors are restricted to single source code files and are the result of 'syntax errors'. What this really means is that you've done something that the compiler cannot understand. For instance, the statement "for(;)" isn't correct syntax because a for loop always needs to have three parts. Although the compiler would have expected a semicolon, it would also have expected a conditional ex