This library does not support comments by default. It does so for three reasons:

1. Comments are not part of the JSON specification. You may argue that // or /* */ are allowed in JavaScript, but JSON is not JavaScript.
2. This was not an oversight: Douglas Crockford wrote on this in May 2012:

I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.

Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.

3. It is dangerous for interoperability if some libraries would add comment support while others don't. Please check The Harmful Consequences of the Robustness Principle on this.

However, you can pass set parameter ignore_comments to true in the parse function to ignore // or /* */ comments. Comments will then be treated as whitespace.

Example

Consider the following JSON with comments.

{
// update in 2006: removed Pluto
"planets": ["Mercury", "Venus", "Earth", "Mars",
"Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
}


When calling parse without additional argument, a parse error exception is thrown. If skip_comments is set to true, the comments are skipped during parsing:

#include <iostream>
#include "json.hpp"

using json = nlohmann::json;

int main()
{
std::string s = R"(
{
// update in 2006: removed Pluto
"planets": ["Mercury", "Venus", "Earth", "Mars",
"Jupiter", "Uranus", "Neptune" /*, "Pluto" */]
}
)";

try
{
json j = json::parse(s);
}
catch (json::exception &e)
{
std::cout << e.what() << std::endl;
}

json j = json::parse(s,
/* callback */ nullptr,
/* allow exceptions */ true,
std::cout << j.dump(2) << '\n';
}


Output:

[json.exception.parse_error.101] parse error at line 3, column 9:
syntax error while parsing object key - invalid literal;
last read: '<U+000A>    {<U+000A>        /'; expected string literal

{
"planets": [
"Mercury",
"Venus",
"Earth",
"Mars",
"Jupiter",
"Uranus",
"Neptune"
]
}