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The std::trunc function is a useful function that returns the integral part of a floating-point number by truncating any fractional part for our C++ program. However, we can often get errors from the compiler such as:

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

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

Potential causes

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

Fix #1: Add cmath to your depedencies

Essentially, the std::trunc 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() {
    double x = -21.14;
    std::cout << std::trunc(x) << std::endl; //Returns -21

    return 0;

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

Fix #2: Using namespace std

Note that we have previously typed: std::trunc instead of trunc. We can type “trunc” 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 trunc (which is obviously shorter) instead of std::trunc. For instance, we can have something like:

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

int main() {
    double x = -21.14;
    cout << trunc(x) << endl; //Returns -21

    return 0;

It is okay to type std::trunc without typing “using namespace std”. In fact, it is generally recommended to type the full std::trunc 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 “trunc” instead of “std::trunc”, then you need to add “using namespace std” to the header.


In this article, we have shown you how to fix the trunc is not a member of std error by including the cmath library, using the std::trunc function and by including “using namespace std” to the header. Always check that you are using the correct library, function name and namespace.


Read more about the std::trunc function here: https://en.cppreference.com/w/cpp/numeric/math/trunc