const vs constexpr on variables

Question

Is there a difference between the following definitions? ``` const double PI = 3.141592653589793; constexpr double PI = 3.141592653589793; ``` If not, which style is preferred in C++11?

Accepted Answer

I believe there is a difference. Let's rename them so that we can talk about them more easily:

```
const     double PI1 = 3.141592653589793;
constexpr double PI2 = 3.141592653589793;
```

Both `PI1` and `PI2` are constant, meaning you can not modify them. However only `PI2` is a compile-time constant. It shall be initialized at compile time. `PI1` may be initialized at compile time or run time. Furthermore, only `PI2` can be used in a context that requires a compile-time constant. For example:

```
constexpr double PI3 = PI1;  // error
```

but:

```
constexpr double PI3 = PI2;  // ok
```

and:

```
static_assert(PI1 == 3.141592653589793, "");  // error
```

but:

```
static_assert(PI2 == 3.141592653589793, "");  // ok
```

As to which you should use? Use whichever meets your needs. Do you want to ensure that you have a compile time constant that can be used in contexts where a compile-time constant is required? Do you want to be able to initialize it with a computation done at run time? Etc.

const vs constexpr on variables

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