nlohmann::basic_json_view::get

template<typename T>
T get() const;

Converts the value to T.

For the types below, the conversion works directly on the flat index -- no BasicJsonType value is built for it:

  • #!cpp bool
  • arithmetic types other than #!cpp bool (for a boolean, #!cpp 0/#!cpp 1, exactly as for BasicJsonType::get<T>())
  • #!cpp std::nullptr_t
  • #!cpp std::basic_string<char, Traits, Alloc> (including string_t) -- a copy of the string
  • string_view_t -- no copy: the returned view points into the document‘s source() text, or, for a string that contains escape sequences, into the document’s own buffer of decoded strings (see get_string())
  • BasicJsonType -- equivalent to materialize()
  • basic_json_view -- returns #!cpp *this
  • #!cpp std::vector<U, A> -- from an array, element by element, each converted with #!cpp get<U>(); #!cpp std::vector<basic_json_view> keeps a view of every element instead of a value
  • #!cpp std::map<K, V, C, A> and #!cpp std::unordered_map<K, V, H, E, A>, if K is constructible from a #!cpp (const char*, std::size_t) pair -- from an object, member by member, each value converted with #!cpp get<V>(); with a repeated key, the last value is kept, as BasicJsonType::parse() (and materialize()) does; #!cpp std::map<std::string, basic_json_view> keeps views of the members instead of values

Every other T -- #!cpp std::list, #!cpp std::pair, #!cpp std::array, enumerations, user types with a from_json(), ... -- is converted by #!cpp materialize().get<T>(): the subtree is built into a real BasicJsonType value first (as BasicJsonType::parse() would), and converted from there exactly as BasicJsonType::get<T>() would convert it.

Template parameters

T : the type to convert the value to

Return value

the value, converted to T

Exception safety

Strong guarantee: there are no changes to the view or the document it refers to.

Exceptions

  • For the directly-converted types listed above (other than BasicJsonType and basic_json_view, which never throw): throws type_error.302 if the value's type does not match T -- the same exception, with the same message, that BasicJsonType::get<T>() throws for the same JSON type and T.
  • For #!cpp std::vector<U, A>: throws type_error.302 if the value is not an array; otherwise, whatever converting an element to U throws.
  • For #!cpp std::map/#!cpp std::unordered_map: throws type_error.302 if the value is not an object; otherwise, whatever converting a member to the mapped type throws.
  • For every other T: whatever materialize().get<T>() throws -- typically type_error.302, or whatever a user-provided from_json() throws.

None of the exceptions thrown directly by this function (the first three bullets above) carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at. An exception thrown while converting through materialize() (the last bullet) is different: it is thrown by a real BasicJsonType value, so it does carry a JSON_DIAGNOSTICS path if BasicJsonType was built with it enabled. That path is relative to the root of the view‘s subtree, because materialize() builds only the subtree (with parent pointers): it is empty if the failing value is the subtree’s root itself, and it never is the path within the whole document.

Complexity

  • #!cpp bool, arithmetic types, #!cpp std::nullptr_t, string_view_t, basic_json_view: constant.
  • #!cpp std::basic_string<char, Traits, Alloc>: constant, plus one allocation and a copy of the string's bytes.
  • BasicJsonType: linear in the size of the subtree, see materialize().
  • #!cpp std::vector<U, A>: linear in the number of elements, times the complexity of converting one element to U.
  • #!cpp std::map/#!cpp std::unordered_map: linear in the number of members for walking them, plus the container's own insertion cost per member (logarithmic for #!cpp std::map, amortized constant for #!cpp std::unordered_map), times the complexity of converting one member to the mapped type.
  • every other T: the cost of materialize() and of destroying the BasicJsonType value afterwards, plus that of BasicJsonType::get<T>().

Notes

!!! info “Floating-point values”

A floating-point `T` is converted from the same digits the lexer would see during `#!cpp BasicJsonType::parse()`,
using the same conversion, so the result is bit-for-bit identical to `#!cpp BasicJsonType::parse(text).get<T>()`
for the same source text.

!!! info “Duplicate keys”

`#!cpp std::map`/`#!cpp std::unordered_map` conversions keep the *last* value of a repeated key, like
[`materialize()`](materialize.md) and [`BasicJsonType::parse()`](../basic_json/parse.md) do. This is the opposite
of [`operator[]`](operator[].md)/[`at`](at.md)/[`find`](find.md)/[`contains`](contains.md), which resolve to the
*first* occurrence (see the [Notes on duplicate keys](operator[].md#notes)).

!!! info “No pointers, references, or implicit conversion”

Unlike `BasicJsonType`, `basic_json_view` has no stored value anywhere to hand out a pointer or a reference to, so
it provides neither `#!cpp get_ptr()`, `#!cpp get_ref()`, nor `#!cpp operator ValueType()`.
[`get_string()`](get_string.md) (equivalently, `#!cpp get<string_view_t>()`) is the zero-copy alternative for
strings.

Examples

??? example

The example below reads typed fields straight into C++ variables, collects a view of every array element with
`#!cpp get<std::vector<basic_json_view>>()` instead of a value, and converts a nested object into a user type
through its `from_json()` -- which runs on a `BasicJsonType` value `materialize()` builds for just that one
member.

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

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

// address has no direct conversion in get<T>(), so get<address>() falls back
// to materialize().get<address>() -- a real nlohmann::json value is built
// for just this one member, and its own from_json() runs on that
struct address
{
    std::string city;
    int zip = 0;
};

void from_json(const nlohmann::json& j, address& a)
{
    j.at("city").get_to(a.city);
    j.at("zip").get_to(a.zip);
}

int main()
{
    json_document doc = json_document::parse(R"(
      {
        "name": "Alice",
        "active": true,
        "orders": [1, 2, 3],
        "address": {"city": "Berlin", "zip": 10115}
      }
    )");
    const json_view customer = doc.root();

    // read typed fields straight into C++ variables -- none of these build
    // a nlohmann::json value
    const std::string name = customer["name"].get<std::string>();
    const bool active = customer["active"].get<bool>();
    std::cout << name << (active ? " (active)" : " (inactive)") << '\n';

    // std::vector<json_view> keeps views of the array elements instead of
    // copies of their values
    bool first = true;
    for (const json_view order : customer["orders"].get<std::vector<json_view>>())
    {
        std::cout << (first ? "" : " ") << order.get<int>();
        first = false;
    }
    std::cout << '\n';

    // everything else goes through materialize()
    const address a = customer["address"].get<address>();
    std::cout << a.city << ' ' << a.zip << '\n';
}

```

Output:

```json
Alice (active)
1 2 3
Berlin 10115

```

See also

Version history

  • Added in version 3.13.0.