nlohmann::basic_json_view::get_to

template<typename T>
T& get_to(T& v) const;

Converts the value to T and stores it in v, in one of two ways:

For the types that get<T>() converts directly -- arithmetic types, #!cpp bool, #!cpp std::nullptr_t, string_view_t, #!cpp std::basic_string, BasicJsonType, basic_json_view, #!cpp std::vector, and #!cpp std::map/#!cpp std::unordered_map with string keys -- v is assigned: #!cpp v = get<T>().
For every other type, the subtree is built into a BasicJsonType value and #!cpp materialize().get_to(v) is called, i.e. from_json() is invoked with the existing v, exactly as BasicJsonType::get_to() does.

Template parameters

T: the type to convert the value to

Parameters

v (out): the variable to store the converted value in

Return value

v, allowing calls to chain

Exception safety

For the directly converted types: strong exception safety. If an exception is thrown, v is not modified.
For every other type: as for BasicJsonType::get_to(). materialize() throws before v is touched, but a from_json() that throws may leave v partly modified.

Exceptions

For the directly converted types: whatever get<T>() throws for the same value and T.
For every other type: whatever materialize() and BasicJsonType::get_to() throw.

Complexity

For the directly converted types: whatever get<T>() has for the same T.
For every other type: that of materialize(), plus that of BasicJsonType::get_to().

Notes

The difference between the two ways matters for a from_json() that sets only the fields present in the JSON: since it runs on the existing v, the other fields of v keep their values, whereas assigning get<T>() would replace them with those of a default-constructed T.

Examples

??? example

The example below reads several fields of a service configuration directly into existing variables, then uses
the returned reference to fold the `#!cpp host`/`#!cpp port` pair into a single string in the same expression.

```cpp
#include <iostream>
#include <nlohmann/json_view.hpp>
#include <string>

using json_document = nlohmann::json_document;
using json_view = nlohmann::json_view;

int main()
{
    json_document doc = json_document::parse(R"({"host": "db.example.com", "port": 5432, "ssl": true})");
    const json_view config = doc.root();

    // get_to() writes directly into existing variables -- handy for filling
    // in the members of a struct one field at a time, without an
    // intermediate value from get<T>() for each one
    std::string host;
    int port = 0;
    bool ssl = false;
    config["host"].get_to(host);
    config["port"].get_to(port);
    config["ssl"].get_to(ssl);
    std::cout << host << ':' << port << (ssl ? " (tls)" : "") << '\n';

    // the return value is a reference to the argument, so a call can be
    // used directly in a larger expression
    std::string other_host;
    std::cout << config["host"].get_to(other_host).size() << '\n';
}

```

Output:

```json
db.example.com:5432 (tls)
14

```

Version history

Added in version 3.13.0.

3.6 KiB Raw Blame History