Type Conversion

[ccge]

Is there a way to convert a string value to a number value or vice versa?

For example:

{
    "number": "9"
}

I figured this word work, but it doesn't:
j.at( "number" ).get<int>();

Do I just have to grab it as a string and convert myself, or is there a better way?

[nlohmann]

The library does not provide such conversions.

You may need something like

#include "json.hpp"

using json = nlohmann::json;

int main()
{
    json j = {{"number", "9"}};
    int i = std::stoi(j["number"].get_ref<std::string&>());
}

[ccge]

Thank you for the quick response. Great library.

[ccge]

I have one more question. What is the difference between?:

j["number"].get_ref<std::string&>()
j["number"]
j.at("number")

They all seem to do the same thing and return a reference.

[gregmarr]

j.at("number") and j["number"] will both return a reference to the json object corresponding to the key "number" if it exists. If not, then at will throw, as will [] on a const j object. For a non-const j, it will create a new key with a null value.

Then .get_ref<std::string&>() returns a reference to the std::string value in the returned object, assuming that is its type.

[ccge]

So, what will tmp be, string& or string?:
auto tmp = j.at("number");

[gregmarr]

This is a copy, auto never deduces to a reference. If you want a reference, use

auto &tmp = j.at("number");

[ccge]

So at and [] can both return a reference or a copy of the value. Why did you say it returns the reference to the key?

[gregmarr]

They always return a reference, which is then copied if you don't use &, because auto deduces json instead of json & or const json &. When you add the &, it deduces json & or const json & depending on whether or not j is const.

[nlohmann]

Can I close this issue?

[nlohmann]

💤 I closed this issue due to inactivity. Please feel free to add a comment and I shall reopen it.

[yalov]

@nlohmann
so, if I got nlohmann::json json, that represent MyStruct, but someone put "value": "5", instead of "value": 5,
then auto myStruct = json.get<MyStruct>() will throw exception?

what would be the proper way to solve this,
implement to_json and from_json manually, instead of one of NLOHMANN_DEFINE_TYPE...() ?
how safe function would looks like (that could work with both int and string) instead of
type_to.val = json_from.value("val", default_obj.val);

Or there is something better, that leaves autogenerating with NLOHMANN_DEFINE_TYPE...() ?