CMake "undefined reference" [duplicate] - c++

Question

Shall this be the example:
#include <iostream>
using namespace std;
int main()
{
cout << "Hola, moondo.\n";
}
It throws the error:
gcc -c main.cpp gcc -o edit main.o main.o: In function `main':
main.cpp:(.text+0xa): undefined reference to `std::cout'
main.cpp:(.text+0xf): undefined reference to `std::basic_ostream<char,std::char_traits<char> >& std::operator<< <std::char_traits<char>>(std::basic_ostream<char, std::char_traits<char> >&, char const*)'
main.o: In function `__static_initialization_and_destruction_0(int,int)':
main.cpp:(.text+0x3d): undefined reference to `std::ios_base::Init::Init()'
main.cpp:(.text+0x4c): undefined reference to `std::ios_base::Init::~Init()' collect2: error: ld
returned 1 exit status make: *** [qs] Error 1
Also, this example:
#include <iostream>
int main()
{
std::cout << "Hola, moondo.\n";
}
throws the error:
gcc -c main.cpp gcc -o edit main.o main.o: In function `main':
main.cpp:(.text+0xa): undefined reference to `std::cout'
main.cpp:(.text+0xf): undefined reference to `std::basic_ostream<char,std::char_traits<char> >& std::operator<<<std::char_traits<char>>(std::basic_ostream<char,std::char_traits<char> >&, char const*)'
main.o: In function `__static_initialization_and_destruction_0(int,int)': main.cpp:(.text+0x3d): undefined reference to `std::ios_base::Init::Init()'
main.cpp:(.text+0x4c): undefined reference to `std::ios_base::Init::~Init()' collect2: error: ld
returned 1 exit status make: *** [qs] Error 1
Note: I am using Debian 7 (Wheezy).

Answer 1

Compile the program with:
g++ -Wall -Wextra -Werror -c main.cpp -o main.o
^^^^^^^^^^^^^^^^^^^^ <- For listing all warnings when your code is compiled.
as cout is present in the C++ standard library, which would need explicit linking with -lstdc++ when using gcc; g++ links the standard library by default.
With gcc, (g++ should be preferred over gcc)
gcc main.cpp -lstdc++ -o main.o

Answer 2

Yes, using g++ command worked for me:
g++ my_source_code.cpp

Answer 3

Assuming code.cpp is the source code, the following will not throw errors:
make code
./code
Here the first command compiles the code and creates an executable with the same name, and the second command runs it. There is no need to specify g++ keyword in this case.

Answer 4

Makefiles
If you're working with a makefile and you ended up here like me, then this is probably what you're looking or:
If you're using a makefile, then you need to change cc as shown below
my_executable : main.o
cc -o my_executable main.o
to
CC = g++
my_executable : main.o
$(CC) -o my_executable main.o

Answer 5

Adding the following line in your CMake makes gcc link with std and therefore recognize std::cout
target_link_libraries(your_project
PRIVATE
-lstdc++
)

Answer 6

FWIW, if you want a makefile, here is how you can do either answer by switching the compiler at the top.
# links stdc++ library by default
# CC := g++
# or
CC := cc
all: hello
util.o: util.cc
$(CC) -c -o util.o util.cc
main.o: main.cc
$(CC) -c -o main.o main.cc
# notice -lstd++ is after the .o files
hello: main.o util.o
$(CC) -o hello main.o util.o -lstdc++
clean:
-rm util.o main.o hello