Floor is not a member of std in C++

Introduction

The std::floor function is a useful function that returns the number rounded downwards for our C++ program. However, we can often get errors from the compiler such as:

error: ‘floor’ is not a member of ‘std’

This error is pretty straight-forward to fix as we will see below.

Potential causes

The floor is not a member of std error could be caused by multiple reasons.

Fix #1: Add iostream to your depedencies

Essentially, the std::floor function needs to have access to the cmath module in order to be executed by the compiler.

Therefore, you must add the following #include header to the top of your code (in the include(s) part) such as:

#include <cmath> //Add this
#include <iostream>

int main() {
    float x = 21.74;
    std::cout << std::floor(x) << std::endl; //Returns 21

    return 0;
}

The compiler should now recognize the std::floor function, thus fixing the floor is not a member of std error.

If the std error still persists, keep reading below.

Fix #2: Using namespace std

Note that we have previously typed: std::floor instead of floor. We can type “floor” only if we are declaring that we are using its namespace.

In other words, we would need to type “using namespace std” in the header if we only want to type floor (which is obviously shorter) instead of std::floor. For instance, we can have something like:

#include <cmath>
#include <iostream>
using namespace std;

int main() {
    float x = 21.74;
    std::cout << std::floor(x) << std::endl; //Returns 21

    return 0;
}

It is okay to type std::floor without typing “using namespace std”. In fact, it is generally recommended to type the full std::floor function name (and therefore avoiding using namespace std) when working with multiple libraries because it can reduce future confusion.

However, if you still want to type “floor” instead of “std::floor”, then you need to add “using namespace std” to the header

References

Read more about the std::floor function here: https://www.cplusplus.com/reference/cmath/floor/