Compile error: 'stoi' is not a member of 'std'

Question

My code:

#include <iostream>
#include <string>

int main()
{
    std::string test = "45";
    int myint = std::stoi(test);
    std::cout << myint << '\n';
}

Gives me the compile error:

error: 'stoi' is not a member of 'std'
     int myint = std::stoi(test);
                 ^

However, according to here, this code should compile fine. I am using the line set(CMAKE_CXX_FLAGS "-std=c++11 -O3") in my CMakeLists.txt file.

Why is it not compiling?

---

Update: I am using gcc, and running gcc --version prints out:

gcc (Ubuntu 5.2.1-22ubuntu2) 5.2.1 20151010

Answer 1

In libstdc++, the definitions of stoi, stol, etc., as well as the to_string functions, are guarded by the condition

#if ((__cplusplus >= 201103L) && defined(_GLIBCXX_USE_C99) \
     && !defined(_GLIBCXX_HAVE_BROKEN_VSWPRINTF))

I have had this fail on one platform before (namely Termux on Android), resulting in to_string not being available even with g++ 6.1 and the C++14 standard. In that case, I just did

#define _GLIBCXX_USE_C99 1

before including anything, and voilà, suddenly the functions existed. (You should put this first, or even on the command line, rather than just before including <string>, because another header may include <string> first, and then its include guards will keep it from ever seeing your macro.)

I did not investigate why this macro wasn't set in the first place. Obviously this is a cause for concern if you want your code to actually work (in my case I didn't particularly, but FWIW there were no problems.)

You should check if _GLIBCXX_USE_C99 is not defined, or if _GLIBCXX_HAVE_BROKEN_VSWPRINTF is defined (which may be the case on MinGW?)

Answer 2

std::stoi is a C++11 function. You have to use the -std=c++11 to enable it in both g++ and clang++. This is the actual issue, not a linking error or a specific preprocessor define.

 $ cat test.cxx
#include <iostream>
#include <string>

int main()
{
    std::string test = "45";
    int myint = std::stoi(test);
    std::cout << myint << '\n';
}
 $ g++ -otest test.cxx
test.cxx: In Funktion »int main()«:
test.cxx:7:17: Fehler: »stoi« ist kein Element von »std«
     int myint = std::stoi(test);
                 ^
 $ g++ -otest test.cxx -std=c++11
 $ ./test
45
 $

edit: I just saw that you used c++11. Are you sure that's making it into your compile options? Check the generated makefile and watch the executed commands to be certain.

Answer 3

Your version seems up to date, so there shouldn't be an issue. I think it may be related to gcc. Try g++ instead.(Most likely automatically linking issue. If you just run gcc on a C++ file, it will not 'just work' like g++ does. That's because it won't automatically link to the C++ std library, etc.). My second advise is try std::atoi.

@ I have fixed the issue. std::stoi uses libstdc++. It is about The GNU Standard C++ Library. In gcc you have to link adding -lstdc++. However, in g++, libstdc++ is linked automatically. using gcc and using g++

Pay attention how it is compiled

using g++: g++ -std=c++11 -O3 -Wall -pedantic main.cpp && ./a.out

using gcc: gcc -std=c++11 -O3 -Wall -pedantic -lstdc++ main.cpp && ./a.out

I think you should set flag like set(CMAKE_EXE_LINKER_FLAGS "-libgcc -lstdc++") (Not tested)

#include <cstdlib>

int myInt = std::atoi(test.c_str());

Answer 4

If you are using Cmake to compile, add line:

"add_definitions(-std=c++11)"

after find_package command.

Answer 5

Use 'set(CMAKE_CXX_STANDARD 11)' for Cmake