std::transform in copying data from json array to a float array C++

August 4, 2023

I have a json array as shown below

{
  { "Data":[1.0, 2.0, 3.0, 4.0] }
}

I am trying to copy this json array to a float array using std::transform.

In code –

#include <nlohmann/json.hpp>
#include <memory>

using json = nlohmann::json;
int main(){
    json tensor;
    tensor["Data"] = { 1, 0, 2 };

    auto dataPtr = std::make_unique<float[]>(3);

    const auto& dataVec = tensor["Data"];
    std::transform(dataVec.cbegin(), dataVec.cend(), dataPtr, [](float data) { return data; });
}

Error –

cannot increment value of type 'std::unique_ptr<float[]>'

Question – How do I use std::transform in this case, where I dont have an iterator

>Solution :

std::transform may not be able to increment the unique_ptr itself, but you can get the contained pointer. The raw pointer is incrementable, so it is usable as a target for the transform:

std::transform(dataVec.cbegin(), dataVec.cend(), dataPtr.get(), [](float data) { return data; });