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Runtime Assertions

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 of, the latter checks for undefined behavior are to detect bugs in client code.

Switch off runtime assertions

Runtime assertions can be switched off by defining the preprocessor macro NDEBUG (see the documentation of assert) which is the default for release builds.

Change assertion behavior

The behavior of runtime assertions can be changes by defining macro JSON_ASSERT(x) before including the json.hpp header.

Function with runtime assertions

Unchecked object access to a const value

Function operator[] implements unchecked access for objects. Whereas a missing key is added in 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.

If you are not sure whether an element in an object exists, use checked access with the at function or call the contains function before.

See also the documentation on element access.

Example 1: Missing object key

The following code will trigger an assertion at runtime:

#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    const json j = {{"key", "value"}};
    auto v = j["missing"];
}

Output:

Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.

Constructing from an uninitialized iterator range

Constructing a JSON value from an iterator range (see constructor) with an uninitialized iterator is undefined behavior and yields a runtime assertion.

Example 2: Uninitialized iterator range

The following code will trigger an assertion at runtime:

#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    json::iterator it1, it2;
    json j(it1, it2);
}

Output:

Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.

Operations on uninitialized iterators

Any operation on uninitialized iterators (i.e., iterators that are not associated with any JSON value) is undefined behavior and yields a runtime assertion.

Example 3: Uninitialized iterator

The following code will trigger an assertion at runtime:

#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
  json::iterator it;
  ++it;
}

Output:

Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.

Reading from a null FILE pointer

Reading from a null FILE pointer is undefined behavior and yields a runtime assertion. This can happen when calling std::fopen on a nonexistent file.

Example 4: Uninitialized iterator

The following code will trigger an assertion at runtime:

#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
  std::FILE* f = std::fopen("nonexistent_file.json", "r");
  json j = json::parse(f);
}

Output:

Assertion failed: (m_file != nullptr), function file_input_adapter, file input_adapters.hpp, line 55.

Last update: September 18, 2022