This is what your code returns:
"TypeError: Result of expression 'fun' [fun(){return 115;}] is not a function."
so your function code is plain wrong 
Here is the proper code:
#include <QtGui>
#include <QtScript>
int main(int argc, char **argv) {
QScriptEngine engine;
QScriptContext *context = engine.pushContext();
QScriptValue v = context->activationObject();
v.setProperty("fun", engine.evaluate("(function(){return 115;})"));
QScriptValue result = engine.evaluate("fun()");
engine.popContext();
qDebug() << result.toString();
return 0;
}
#include <QtGui>
#include <QtScript>
int main(int argc, char **argv) {
QApplication app(argc, argv);
QScriptEngine engine;
QScriptContext *context = engine.pushContext();
QScriptValue v = context->activationObject();
v.setProperty("fun", engine.evaluate("(function(){return 115;})"));
QScriptValue result = engine.evaluate("fun()");
engine.popContext();
qDebug() << result.toString();
return 0;
}
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