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<span class=w> </span><span class=p>}</span>
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<span class=p>}</span>
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</code></pre></div> <p>Output:</p> <div class=highlight><pre><span></span><code>[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/macros/json_assert/ >JSON_ASSERT</a> - control behavior of runtime assertions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="July 4, 2026 09:13:25 UTC">July 4, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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</code></pre></div> </details> <h2 id=see-also>See also<a class=headerlink href=#see-also title="Permanent link">¶</a></h2> <ul> <li><a href=../../api/macros/json_assert/ >JSON_ASSERT</a> - control behavior of runtime assertions</li> </ul> <!-- https://squidfunk.github.io/mkdocs-material/reference/tooltips/#adding-a-glossary --> <aside class=md-source-file> <span class=md-source-file__fact> <span class=md-icon title="Last update"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M21 13.1c-.1 0-.3.1-.4.2l-1 1 2.1 2.1 1-1c.2-.2.2-.6 0-.8l-1.3-1.3c-.1-.1-.2-.2-.4-.2m-1.9 1.8-6.1 6V23h2.1l6.1-6.1zM12.5 7v5.2l4 2.4-1 1L11 13V7zM11 21.9c-5.1-.5-9-4.8-9-9.9C2 6.5 6.5 2 12 2c5.3 0 9.6 4.1 10 9.3-.3-.1-.6-.2-1-.2s-.7.1-1 .2C19.6 7.2 16.2 4 12 4c-4.4 0-8 3.6-8 8 0 4.1 3.1 7.5 7.1 7.9l-.1.2z"/></svg> </span> <span class="git-revision-date-localized-plugin git-revision-date-localized-plugin-date" title="July 8, 2026 18:18:24 UTC">July 8, 2026</span> </span> </aside> </article> </div> <script>var tabs=__md_get("__tabs");if(Array.isArray(tabs))e:for(var set of document.querySelectorAll(".tabbed-set")){var labels=set.querySelector(".tabbed-labels");for(var tab of tabs)for(var label of labels.getElementsByTagName("label"))if(label.innerText.trim()===tab){var input=document.getElementById(label.htmlFor);input.checked=!0;continue e}}</script> <script>var target=document.getElementById(location.hash.slice(1));target&&target.name&&(target.checked=target.name.startsWith("__tabbed_"))</script> </div> <button type=button class="md-top md-icon" data-md-component=top hidden> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 24 24"><path d="M13 20h-2V8l-5.5 5.5-1.42-1.42L12 4.16l7.92 7.92-1.42 1.42L13 8z"/></svg> Back to top </button> </main> <footer class=md-footer> <div class="md-footer-meta md-typeset"> <div class="md-footer-meta__inner md-grid"> <div class=md-copyright> <div class=md-copyright__highlight> Copyright © 2013-2026 Niels Lohmann </div> </div> <div class=md-social> <a href="https://github.com/nlohmann" target="_blank" rel="noopener" title="github.com" class="md-social__link"> <svg xmlns=http://www.w3.org/2000/svg viewbox="0 0 512 512"><!-- Font Awesome Free 7.1.0 by @fontawesome - 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@@ -0,0 +1,132 @@
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# Runtime Assertions
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The code contains numerous debug assertions to ensure class invariants are valid or to detect undefined behavior. Whereas the former class invariants are nothing to be concerned with, the latter checks for undefined behavior are to detect bugs in client code.
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## Switch off runtime assertions
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Runtime assertions can be switched off by defining the preprocessor macro `NDEBUG` (see the [documentation of assert](https://en.cppreference.com/w/cpp/error/assert)) which is the default for release builds.
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## Change assertion behavior
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The behavior of runtime assertions can be changed by defining macro [`JSON_ASSERT(x)`](https://json.nlohmann.me/api/macros/json_assert/index.md) before including the `json.hpp` header.
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## Function with runtime assertions
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### Unchecked object access to a const value
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Function [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md) implements unchecked access for objects. Whereas a missing key is added in the case of non-const objects, accessing a const object with a missing key is undefined behavior (think of a dereferenced null pointer) and yields a runtime assertion.
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If you are not sure whether an element in an object exists, use checked access with the [`at` function](https://json.nlohmann.me/api/basic_json/at/index.md) or call the [`contains` function](https://json.nlohmann.me/api/basic_json/contains/index.md) before.
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See also the documentation on [element access](https://json.nlohmann.me/features/element_access/index.md).
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Example 1: Missing object key
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The following code will trigger an assertion at runtime:
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```
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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const json j = {{"key", "value"}};
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auto v = j["missing"];
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}
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```
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Output:
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```
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Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.
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```
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### Constructing from an uninitialized iterator range
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Constructing a JSON value from an iterator range (see [constructor](https://json.nlohmann.me/api/basic_json/basic_json/index.md)) with an uninitialized iterator is undefined behavior and yields a runtime assertion.
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Example 2: Uninitialized iterator range
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The following code will trigger an assertion at runtime:
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```
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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json::iterator it1, it2;
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json j(it1, it2);
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}
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```
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Output:
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```
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Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
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```
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### Operations on uninitialized iterators
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Any operation on uninitialized iterators (i.e., iterators that are not associated with any JSON value) is undefined behavior and yields a runtime assertion.
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Example 3: Uninitialized iterator
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The following code will trigger an assertion at runtime:
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```
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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{
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json::iterator it;
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++it;
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}
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```
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Output:
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```
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Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.
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```
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## Changes
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### Reading from a null `FILE` or `char` pointer
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Reading from a null `FILE` or `char` pointer in C++ is undefined behavior. Until version 3.12.0, this library asserted that the pointer was not `nullptr` using a runtime assertion. If assertions were disabled, this would result in undefined behavior. Since version 3.12.0, this library checks for `nullptr` and throws a [`parse_error.101`](https://json.nlohmann.me/home/exceptions/#jsonexceptionparse_error101) to prevent the undefined behavior.
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Example 4: Reading from null pointer
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The following code will trigger an assertion at runtime:
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```
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#include <iostream>
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#include <nlohmann/json.hpp>
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using json = nlohmann::json;
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int main()
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||||
{
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std::FILE* f = std::fopen("nonexistent_file.json", "r");
|
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try {
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json j = json::parse(f);
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} catch (std::exception& e) {
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std::cerr << e.what() << std::endl;
|
||||
}
|
||||
}
|
||||
```
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||||
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Output:
|
||||
|
||||
```
|
||||
[json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON
|
||||
```
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||||
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||||
## See also
|
||||
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||||
- [JSON_ASSERT](https://json.nlohmann.me/api/macros/json_assert/index.md) - control behavior of runtime assertions
|
||||
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