jsoncpp如何检查标记是否为null .isnull（）抛出断言

Question

IM使用jsoncpp，它很棒，但是当我需要检查json结构是否包含标记 当我用的时候：

UserRoot0["error"].isNull()

.

它抛出了json_value.cpp行1025

JSON_ASSERT( type_ == nullValue  ||  type_ == objectValue );

.

我想检查响应im get是从此类型的：

{
    "error" : {
        "message" : "Error validating application.",
        "type" : "OAuthException",
        "code" : 190
    }
}

.

Answer 1

The [] operator is only valid for JsonValue objects that are of type Object or null. All others (Int, Bool, Array, etc.) will assert.

If your UserRoot0 object is an Array or some other non-Object type, you have some more work to do (like iterating into sub-nodes) to find your target node that may or may not contain the error. Print UserRoot0.toStyledString() to see what your JSON looks like, and make sure it looks like a JSON Object (see json.org for a nice overview of what that is).

A "ToDo" comment at the top of the json_value.cpp source file (where JSON_ASSERT is defined) implies that the developers may be planning more robust error handling instead of these asserts in future versions, but in the meantime, you can check yourself, like this:

if(UserRoot0.isObject() && UserRoot0.isMember("error"))
    // Process error node
else
    // This node isn't an Object node or doesn't contain the "error" key

The isMember() check will also assert for non-Object nodes, so be sure to check isObject() before checking isMember() if UserRoot0 isn't guaranteed to be an Object.

Answer 2

I stumbled on this too. As Ennael says you need to make sure you are dealing with an object type. FWIW my problem was caused by using JSON like this:

    "error" : {
        "code" : 190
    }

... instead of what I intended:

{
    "error" : {
        "code" : 190
    }
}

Excluding the outer set of parenthesis will cause the value type to become string instead of object.