Reusing a float buffer for doubles without undefined behaviour

Question

In one particular C++ function, I happen to have a pointer to a big buffer of floats that I want to temporarily use to store half the number of doubles. Is there a method to use

Answer 1

You can achieve this in two ways.

First:

void set(float *buffer, size_t index, double value) {
    memcpy(reinterpret_cast(buffer)+sizeof(double)*index, &value, sizeof(double));
}
double get(const float *buffer, size_t index) {
    double v;
    memcpy(&v, reinterpret_cast(buffer)+sizeof(double)*index, sizeof(double));
    return v;
}
void f(float *buffer) {
    // here, use set and get functions
}

Second: Instead of float *, you need to allocate a "typeless" char[] buffer, and use placement new to put floats or doubles inside:

template 
void setType(char *buffer, size_t size) {
    for (size_t i=0; i(buffer, sizeOfBuffer);

Then use this accessor:

template 
T &get(char *buffer, size_t index) {
    return *std::launder(reinterpret_cast(buffer+index*sizeof(T)));
}
// use it like this: get(buffer, index) = 33.3f;

A third way could be something like phön's answer (see my comments under that answer), unfortunately I cannot make a proper solution, because of this problem.