---
title: QkComplex64 (latest version)
description: API reference for QkComplex64 in the latest version of qiskit-c
source: https://quantum.cloud.ibm.com/docs/en/api/qiskit-c/qk-complex-64
---

# QkComplex64

A complex, double-precision number representation. This data type is used to safely use complex numbers within Qiskit, but is not necessarily designed for easy manipulation on user-side. Instead, we provide functions to convert from compiler-native representation (e.g. `complex double` for GNU or Clang compilers), which allow for ergonomic handling, to Qiskit’s `QkComplex64` representation, which is meant for passing into Qiskit functions and structs.

Explicitly, Qiskit assumes the compiler-native complex number type in C to be `_Dcomplex` for Windows MSVC (if `_MSC_VER` is defined) and `double _Complex` otherwise. In C++ (if `__cplusplus` is defined), the complex number type is always `std::complex<double>`.

## Data types

### QkComplex64

`struct QkComplex64`

A complex double.

See also `qk_complex64_to_native` and `qk_complex64_from_native` to convert this struct to (or from) a compiler-native complex number representation.

#### double re

The real part.

#### double im

The imaginary part.

## Functions

### qk\_complex64\_to\_native

`static CMPLX_DOUBLE qk_complex64_to_native(QkComplex64 *value)`

Convert a `QkComplex64` to a compiler-native complex number representation. Note that `CMPLX_DOUBLE` is a placeholder for the compiler-native complex number type, see the top of the page for more details.

### Example

Assuming a GNU/clang compiler with `complex double` as native complex number, we have

```c
QkComplex64 qk_value = {1, 1}; // represents 1 + i
complex double value = qk_complex64_to_native(&qk_value);
```

### Safety

Behavior is undefined if `value` is not a valid, non-null pointer to a `QkComplex64`.

**Parameters**

**value** – A pointer to the `QkComplex64` to convert.

**Returns**

A native representation of the complex number.

#### qk\_complex64\_from\_native

`static QkComplex64 qk_complex64_from_native(CMPLX_DOUBLE *value)`

Convert a compiler-native complex number to a `QkComplex64`. Note that `CMPLX_DOUBLE` is a placeholder for the compiler-native complex number type, see the top of the page for more details.

### Example

Assuming a GNU/clang compiler with `complex double` as native complex number, we have

```c
complex double value = 1 + I;
QkComplex64 qk_value = qk_complex64_from_native(&value);
```

### Safety

Behavior is undefined if `value` is not a valid, non-null pointer to a `QkComplex64`.

**Parameters**

**value** – A pointer to the native complex number.

**Returns**

A `QkComplex64` representation of the complex number.