Error map
incomplete type C++: error: invalid use of incomplete type 'class X' — forward declaration only
C/C++ · layer 4, Application & API
Centre: this error. First ring: errors it is linked with (8). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (8)
C++: error: redefinition of 'class X' / multiple definition — missing include guard
The same class, struct or function body was included twice in one translation unit because the header has no include guard.
C++: error: use of deleted function 'X::X(const X&)' — copying a non-copyable type
The code needs a function (usually a copy constructor or assignment) that was deleted explicitly or implicitly, as for `std::uniq…
C++: error: 'string' does not name a type
Where the compiler expected a type, it found a name it does not know as one — a missing header, namespace, or class declared late…
C++: error: cannot declare variable 'x' to be of abstract type 'Shape' (pure virtual functions)
You tried to create an object of a class that still has pure virtual functions; only a derived class that overrides all of them c…
C++: error: 'int X::secret' is private within this context
Code outside the class tried to use a private (or protected) member.
C++: error: passing 'const X' as 'this' argument discards qualifiers [-fpermissive]
A const object (or const reference) is calling a method that is not declared const, which could modify it.
C: warning: implicit declaration of function 'foo' [-Wimplicit-function-declaration]
In C you called a function the compiler has not seen declared — usually a missing `#include`. Newer compilers treat it as an erro…
C/C++: multiple definition of `x'; first defined here — ld returned 1 exit status
The linker found the same function or global variable defined in two object files — usually something defined in a header.
Two steps away (21)
C/C++: undefined reference to `function' — collect2: error: ld returned 1 exit status
The linker can’t find the definition of a function or variable that was declared and used.
C/C++: fatal error: foo.h: No such file or directory — #include not found
The preprocessor cannot find a header named in `#include`: wrong path, missing include directory (-I), or the library is not inst…
C++: error: 'optional' in namespace 'std' does not name a template type — wrong -std standard
The standard-library feature you used needs a newer C++ standard than the compiler is currently set to.
C/C++: free(): double free detected / double free or corruption (out) — Aborted (core dumped)
Memory was freed twice or the heap was corrupted (buffer overflow, freeing a non-heap pointer), so glibc aborted the program.
C++: error: no matching function for call to 'f(int, const char*)'
No overload of the function accepts the arguments you passed — wrong count, wrong types, or a missing conversion.
C++: error: call of overloaded 'f(int)' is ambiguous
Two or more overloads fit equally well, so the compiler refuses to guess which function you meant.
Go: declared and not used: x / "fmt" imported and not used
Go refuses to compile code with unused local variables or unused imports.
Rust: error[E0308]: mismatched types — expected `X`, found `Y`
A value’s type doesn’t match what the context expects — common with &str vs String, Option/Result vs plain values, and integer ty…
C/C++: error: expected ';' before 'x' — missing semicolon
The compiler reached a token where a statement should have ended — almost always a missing semicolon on the line above.
C++: error: no match for 'operator<<' (operand types are 'std::ostream' and 'X')
You used an operator on a type that does not define it — most often printing your own class with `
C/C++: error: stray '\342' in program (smart quotes or invisible characters)
The source file contains a character the compiler does not recognise, typically curly quotes or a non-breaking space pasted from…
C/C++: warning: control reaches end of non-void function [-Wreturn-type]
A function that promises a return value can finish without returning one on some path — undefined behaviour in C++.
C/C++: ==ERROR: AddressSanitizer: heap-buffer-overflow on address — READ/WRITE of size N
AddressSanitizer caught a read or write just outside a heap block — an array index or length that is off by one or more.
C/C++: ==ERROR: AddressSanitizer: heap-use-after-free on address — memory used after delete/free
The program read or wrote a heap block after it had been freed; ASan shows who freed it and where it was allocated.
C++: error: invalid conversion from 'const char*' to 'char*' [-fpermissive]
C++ forbids silently dropping `const` or converting between unrelated pointer types — typically a string literal assigned to `cha…
C++: error: 'x' was not declared in this scope (undeclared identifier)
The compiler met a name it has never seen: it is misspelled, declared later, out of scope, or its header or namespace is missing.
C/C++: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' [-Wsign-compare]
A signed integer is compared with an unsigned one; the signed value is converted to unsigned, so a negative number becomes huge.
C: warning: format '%d' expects argument of type 'int', but argument 2 has type 'long int' [-Wformat=]
The `printf`/`scanf` format specifier does not match the argument's type, which is undefined behaviour and prints garbage or cras…
C++: undefined reference to `vtable for X' — virtual function not defined
The linker cannot find the class's virtual table, which means at least one virtual member function was declared but never defined…
C/C++: undefined reference to `main' — (.text+0x20) in crt1.o
The linker has no `main` function: the file with `main` is not compiled or linked, or `main` is spelled or declared incorrectly.
C/C++: undefined reference to `pthread_create' — missing -pthread (also `dlopen`, `clock_gettime`)
Your program uses threads (or another system library) but the linker was not told to link it.
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