QScriptContext Class | Qt Script

Qt 5.6 | Qt Script | C++ Classes | QScriptContext

Qt 5.6.2 Reference Documentation

# QScriptContext Class

The QScriptContext class represents a Qt Script function invocation. More...

Header: | #include <QScriptContext>
qmake: | QT += script
Since: | Qt 4.3

- List of all members, including inherited members

## Public Types

enum | Error { ReferenceError, SyntaxError, TypeError, RangeError, URIError, UnknownError }
enum | ExecutionState { NormalState, ExceptionState }

## Public Functions

| ~QScriptContext()
QScriptValue | activationObject() const
QScriptValue | argument(int index) const
int | argumentCount() const
QScriptValue | argumentsObject() const
QStringList | backtrace() const
QScriptValue | callee() const
QScriptEngine * | engine() const
bool | isCalledAsConstructor() const
QScriptContext * | parentContext() const
void | setActivationObject(const QScriptValue &activation)
void | setThisObject(const QScriptValue &thisObject)
ExecutionState | state() const
QScriptValue | thisObject() const
QScriptValue | throwError(Error error, const QString &text)
QScriptValue | throwError(const QString &text)
QScriptValue | throwValue(const QScriptValue &value)
QString | toString() const

## Detailed Description

The QScriptContext class represents a Qt Script function invocation.

A QScriptContext provides access to the `this' object and arguments passed to a script function. You typically want to access this information when you're writing a native (C++) function (see QScriptEngine::newFunction()) that will be called from script code. For example, when the script code

      foo(20.5, "hello", new Object())

is evaluated, a QScriptContext will be created, and the context will carry the arguments as QScriptValues; in this particular case, the arguments will be one QScriptValue containing the number 20.5, a second QScriptValue containing the string `"hello"`, and a third QScriptValue containing a Qt Script object.

Use argumentCount() to get the number of arguments passed to the function, and argument() to get an argument at a certain index. The argumentsObject() function returns a Qt Script array object containing all the arguments; you can use the QScriptValueIterator to iterate over its elements, or pass the array on as arguments to another script function using QScriptValue::call().

Use thisObject() to get the `this' object associated with the function call, and setThisObject() to set the `this' object. If you are implementing a native "instance method", you typically fetch the thisObject() and access one or more of its properties:

      QScriptValue Person_prototype_fullName(QScriptContext *context, QScriptEngine *engine)
      {
          QScriptValue self = context->thisObject();
          QString result;
          result += self.property("firstName").toString();
          result += QLatin1String(" ");
          result += self.property("lastName").toString();
          return result;
      }

Use isCalledAsConstructor() to determine if the function was called as a constructor (e.g. `"new foo()"` (as constructor) or just `"foo()"`). When a function is called as a constructor, the thisObject() contains the newly constructed object that the function is expected to initialize.

Use throwValue() or throwError() to throw an exception.

Use callee() to obtain the QScriptValue that represents the function being called. This can for example be used to call the function recursively.

Use parentContext() to get a pointer to the context that precedes this context in the activation stack. This is mostly useful for debugging purposes (e.g. when constructing some form of backtrace).

The activationObject() function returns the object that is used to hold the local variables associated with this function call. You can replace the activation object by calling setActivationObject(). A typical usage of these functions is when you want script code to be evaluated in the context of the parent context, e.g. to implement an include() function:

      QScriptValue myInclude(QScriptContext *ctx, QScriptEngine *eng)
      {
          QString fileName = ctx->argument(0).toString();
          QString contents = readTheFile(fileName);
          ctx->setActivationObject(ctx->parentContext()->activationObject());
          ctx->setThisObject(ctx->parentContext()->thisObject());
          return eng->evaluate(contents, fileName);
      }

Use backtrace() to get a human-readable backtrace associated with this context. This can be useful for debugging purposes when implementing native functions. The toString() function provides a string representation of the context. (QScriptContextInfo provides more detailed debugging-related information about the QScriptContext.)

Use engine() to obtain a pointer to the QScriptEngine that this context resides in.

See also QScriptContextInfo, QScriptEngine::newFunction(), and QScriptable.

## Member Type Documentation

### enum QScriptContext::Error

This enum specifies types of error.

Constant | Value | Description
--- | --- | ---
`QScriptContext::ReferenceError` | `1` | A reference error.
`QScriptContext::SyntaxError` | `2` | A syntax error.
`QScriptContext::TypeError` | `3` | A type error.
`QScriptContext::RangeError` | `4` | A range error.
`QScriptContext::URIError` | `5` | A URI error.
`QScriptContext::UnknownError` | `0` | An unknown error.

### enum QScriptContext::ExecutionState

This enum specifies the frameution state of the context.

Constant | Value | Description
--- | --- | ---
`QScriptContext::NormalState` | `0` | The context is in a normal state.
`QScriptContext::ExceptionState` | `1` | The context is in an exceptional state.

## Member Function Documentation

### QScriptContext::~QScriptContext()

Destroys this QScriptContext.

### QScriptValue QScriptContext::activationObject() const

Returns the activation object of this QScriptContext. The activation object provides access to the local variables associated with this context.

Note: The activation object might not be available if there is no active QScriptEngineAgent, as it might be optimized.

See also setActivationObject(), argument(), and argumentsObject().

### QScriptValue QScriptContext::argument(int index) const

Returns the function argument at the given index.

If index >= argumentCount(), a QScriptValue of the primitive type Undefined is returned.

See also argumentCount().

### int QScriptContext::argumentCount() const

Returns the number of arguments passed to the function in this invocation.

Note that the argument count can be different from the formal number of arguments (the `length` property of callee()).

See also argument().

### QScriptValue QScriptContext::argumentsObject() const

Returns the arguments object of this QScriptContext.

The arguments object has properties `callee` (equal to callee()) and `length` (equal to argumentCount()), and properties `0`, `1`, ..., argumentCount() - 1 that provide access to the argument values. Initially, property `P` (0 <= `P` < argumentCount()) has the same value as argument(`P`). In the case when `P` is less than the number of formal parameters of the function, `P` shares its value with the corresponding property of the activation object (activationObject()). This means that changing this property changes the corresponding property of the activation object and vice versa.

See also argument() and activationObject().

### QStringList QScriptContext::backtrace() const

Returns a human-readable backtrace of this QScriptContext.

Each line is of the form `<function-name>(<arguments>)@<file-name>:<line-number>`.

To access individual pieces of debugging-related information (for example, to construct your own backtrace representation), use QScriptContextInfo.

See also QScriptEngine::uncaughtExceptionBacktrace(), QScriptContextInfo, and toString().

### QScriptValue QScriptContext::callee() const

Returns the callee. The callee is the function object that this QScriptContext represents an invocation of.

### QScriptEngine *QScriptContext::engine() const

Returns the QScriptEngine that this QScriptContext belongs to.

### bool QScriptContext::isCalledAsConstructor() const

Returns true if the function was called as a constructor (e.g. `"new foo()"`); otherwise returns false.

When a function is called as constructor, the thisObject() contains the newly constructed object to be initialized.

Note: This function is only guaranteed to work for a context corresponding to native functions.

### QScriptContext *QScriptContext::parentContext() const

Returns the parent context of this QScriptContext.

### void QScriptContext::setActivationObject(const QScriptValue &activation)

Sets the activation object of this QScriptContext to be the given activation.

If activation is not an object, this function does nothing.

Note: For a context corresponding to a JavaScript function, this is only guaranteed to work if there was an QScriptEngineAgent active on the engine while the function was evaluated.

See also activationObject().

### void QScriptContext::setThisObject(const QScriptValue &thisObject)

Sets the `this' object associated with this QScriptContext to be thisObject.

If thisObject is not an object, this function does nothing.

See also thisObject().

### ExecutionState QScriptContext::state() const

Returns the frameution state of this QScriptContext.

### QScriptValue QScriptContext::thisObject() const

Returns the `this' object associated with this QScriptContext.

See also setThisObject().

### QScriptValue QScriptContext::throwError(Error error, const QString &text)

Throws an error with the given text. Returns the created error object.

The text will be stored in the `message` property of the error object.

The error object will be initialized to contain information about the location where the error occurred; specifically, it will have properties `lineNumber`, `fileName` and `stack`. These properties are described in Qt Script Extensions to ECMAScript.

See also throwValue() and state().

### QScriptValue QScriptContext::throwError(const QString &text)

This is an overloaded function.

Throws an error with the given text. Returns the created error object.

See also throwValue() and state().

### QScriptValue QScriptContext::throwValue(const QScriptValue &value)

Throws an exception with the given value. Returns the value thrown (the same as the argument).

See also throwError() and state().

### QString QScriptContext::toString() const

Returns a string representation of this context. This is useful for debugging.

This function was introduced in Qt 4.4.

See also backtrace().