api/basic_json/push_back/index.md

# nlohmann::basic_json::push_back

```
// (1)
void push_back(basic_json&& val);
void push_back(const basic_json& val);

// (2)
void push_back(const typename object_t::value_type& val);

// (3)
void push_back(initializer_list_t init);
```

1. Appends the given element `val` to the end of the JSON array. If the function is called on a JSON null value, an empty array is created before appending `val`.

1. Inserts the given element `val` to the JSON object. If the function is called on a JSON null value, an empty object is created before inserting `val`.

1. This function allows using `push_back` with an initializer list. In case

   1. the current value is an object,
   1. the initializer list `init` contains only two elements, and
   1. the first element of `init` is a string,

   `init` is converted into an object element and added using `push_back(const typename object_t::value_type&)`. Otherwise, `init` is converted to a JSON value and added using `push_back(basic_json&&)`.

## Iterator invalidation

For all cases where an element is added to an **array**, a reallocation can happen, in which case all iterators (including the [`end()`](https://json.nlohmann.me/api/basic_json/end/index.md) iterator) and all references to the elements are invalidated. Otherwise, only the [`end()`](https://json.nlohmann.me/api/basic_json/end/index.md) iterator is invalidated.

For [`ordered_json`](https://json.nlohmann.me/api/ordered_json/index.md), also adding an element to an **object** can yield a reallocation which again invalidates all iterators and all references.

## Parameters

`val` (in) : the value to add to the JSON array/object

`init` (in) : an initializer list

## Exceptions

1. Throws [`type_error.308`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error308) when called on a type other than JSON array or null; example: `"cannot use push_back() with number"`
1. Throws [`type_error.308`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error308) when called on a type other than JSON object or null; example: `"cannot use push_back() with number"`
1. Throws [`type_error.308`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error308) when called on a type other than JSON array or null; example: `"cannot use push_back() with number"`

## Complexity

1. Amortized constant.
1. Logarithmic in the size of the container, O(log(`size()`)).
1. Linear in the size of the initializer list `init`.

## Notes

(3) This function is required to resolve an ambiguous overload error, because pairs like `{"key", "value"}` can be both interpreted as `object_t::value_type` or `std::initializer_list<basic_json>`, see [#235](https://github.com/nlohmann/json/issues/235) for more information.

## Examples

Example: (1) add element to array

The example shows how `push_back()` and `+=` can be used to add elements to a JSON array. Note how the `null` value was silently converted to a JSON array.

```
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON values
    json array = {1, 2, 3, 4, 5};
    json null;

    // print values
    std::cout << array << '\n';
    std::cout << null << '\n';

    // add values
    array.push_back(6);
    array += 7;
    null += "first";
    null += "second";

    // print values
    std::cout << array << '\n';
    std::cout << null << '\n';
}
```

Output:

```
[1,2,3,4,5]
null
[1,2,3,4,5,6,7]
["first","second"]
```

Example: (2) add element to object

The example shows how `push_back()` and `+=` can be used to add elements to a JSON object. Note how the `null` value was silently converted to a JSON object.

```
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON values
    json object = {{"one", 1}, {"two", 2}};
    json null;

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';

    // add values
    object.push_back(json::object_t::value_type("three", 3));
    object += json::object_t::value_type("four", 4);
    null += json::object_t::value_type("A", "a");
    null += json::object_t::value_type("B", "b");

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';
}
```

Output:

```
{"one":1,"two":2}
null
{"four":4,"one":1,"three":3,"two":2}
{"A":"a","B":"b"}
```

Example: (3) add to object from initializer list

The example shows how initializer lists are treated as objects when possible.

```
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // create JSON values
    json object = {{"one", 1}, {"two", 2}};
    json null;

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';

    // add values:
    object.push_back({"three", 3});  // object is extended
    object += {"four", 4};           // object is extended
    null.push_back({"five", 5});     // null is converted to array

    // print values
    std::cout << object << '\n';
    std::cout << null << '\n';

    // would throw:
    //object.push_back({1, 2, 3});
}
```

Output:

```
{"one":1,"two":2}
null
{"four":4,"one":1,"three":3,"two":2}
[["five",5]]
```

## See also

- [emplace_back](https://json.nlohmann.me/api/basic_json/emplace_back/index.md) add a value to an array
- [operator+=](https://json.nlohmann.me/api/basic_json/operator%2B%3D/index.md) add a value to an array/object
- [Modifying values](https://json.nlohmann.me/features/modifying_values/index.md) - the article on modifying values

## Version history

1. Since version 1.0.0.
1. Since version 1.0.0.
1. Since version 2.0.0.