Qt Lambda reference

SPlatten

I have a Lambda slot:

cnSignal = QObject::connect(this
                           ,&clsQtTextEdit::jsonTextChanged
                           ,[pobjScriptEng, strCall, strFile, strScript, strSID](QJsonObject objJSON) {
                               objJSON.insert(clsXMLnode::mscszAttrSubscriberID, strSID);
                               QString strJSON = clsXMLnode::strJSON(objJSON)
                                      ,strScriptWithCall = static_cast<QString>(strScript)
                                                                        + static_cast<QString>(strCall)
                                                                        + static_cast<QString>("(" + strJSON + ");");
                               pobjScriptEng->evaluate(strScriptWithCall);
                           });

I have multiple connections to different signals that all share the same Lambda function, is there any way to defined a local single instance of this and assign a reference or pointer to that instance instead of replication the same logic in multiple places?

I'm thinking like a function pointer.

Bonnie

@SPlatten

If you only want use that lambda in the current scope. I think auto is the best because it is very simple.

auto func = [pobjScriptEng, strCall, strFile, strScript, strSID](QJsonObject objJSON) {
                               objJSON.insert(clsXMLnode::mscszAttrSubscriberID, strSID);
                               QString strJSON = clsXMLnode::strJSON(objJSON)
                                      ,strScriptWithCall = static_cast<QString>(strScript)
                                                                        + static_cast<QString>(strCall)
                                                                        + static_cast<QString>("(" + strJSON + ");");
                               pobjScriptEng->evaluate(strScriptWithCall);
                           };
cnSignal = QObject::connect(this , &clsQtTextEdit::jsonTextChanged , func);

But if you need to assign the lambda to a class member, then you need std::function because you cannot define an auto member variable.

std::function<void(QJsonObject)> func = [pobjScriptEng, strCall, strFile, strScript, strSID](QJsonObject objJSON) {
                               objJSON.insert(clsXMLnode::mscszAttrSubscriberID, strSID);
                               QString strJSON = clsXMLnode::strJSON(objJSON)
                                      ,strScriptWithCall = static_cast<QString>(strScript)
                                                                        + static_cast<QString>(strCall)
                                                                        + static_cast<QString>("(" + strJSON + ");");
                               pobjScriptEng->evaluate(strScriptWithCall);
                           };
cnSignal = QObject::connect(this , &clsQtTextEdit::jsonTextChanged , func);

JonB

@SPlatten
I believe you are asking to be able to create & re-use a lambda "variable", right? I looked into this a couple of weeks ago. From what I read, C++ lambda object-variables are not so bad to write if they do not take any parameters. But if they do --- your QJsonObject objJSON --- then it's not so easy. If you persist with that, I read you must go via std::function() to do the job. An alternative suggestion was to move any parameters to the [...] context instead of passing as a function parameter.

I gave up on it as being too much hassle. But I shall watch this question to see whether one of our resident C++ experts wants to show just how easily it can be done....

Bonnie

I think this is a C++11 question rather than a Qt one.
The key points here are auto and std::function.
You can refer to this example from https://en.cppreference.com/w/cpp/language/lambda :

#include <vector>
#include <iostream>
#include <algorithm>
#include <functional>
 
int main()
{
    std::vector<int> c = {1, 2, 3, 4, 5, 6, 7};
    int x = 5;
    c.erase(std::remove_if(c.begin(), c.end(), [x](int n) { return n < x; }), c.end());
 
    std::cout << "c: ";
    std::for_each(c.begin(), c.end(), [](int i){ std::cout << i << ' '; });
    std::cout << '\n';
 
    // the type of a closure cannot be named, but can be inferred with auto
    // since C++14, lambda could own default arguments
    auto func1 = [](int i = 6) { return i + 4; };
    std::cout << "func1: " << func1() << '\n';
 
    // like all callable objects, closures can be captured in std::function
    // (this may incur unnecessary overhead)
    std::function<int(int)> func2 = [](int i) { return i + 4; };
    std::cout << "func2: " << func2(6) << '\n';
}

SPlatten

@Bonnie , thank you, do you have an example on how to apply that technique to my example slot?

Bonnie

@SPlatten

If you only want use that lambda in the current scope. I think auto is the best because it is very simple.

auto func = [pobjScriptEng, strCall, strFile, strScript, strSID](QJsonObject objJSON) {
                               objJSON.insert(clsXMLnode::mscszAttrSubscriberID, strSID);
                               QString strJSON = clsXMLnode::strJSON(objJSON)
                                      ,strScriptWithCall = static_cast<QString>(strScript)
                                                                        + static_cast<QString>(strCall)
                                                                        + static_cast<QString>("(" + strJSON + ");");
                               pobjScriptEng->evaluate(strScriptWithCall);
                           };
cnSignal = QObject::connect(this , &clsQtTextEdit::jsonTextChanged , func);

But if you need to assign the lambda to a class member, then you need std::function because you cannot define an auto member variable.

std::function<void(QJsonObject)> func = [pobjScriptEng, strCall, strFile, strScript, strSID](QJsonObject objJSON) {
                               objJSON.insert(clsXMLnode::mscszAttrSubscriberID, strSID);
                               QString strJSON = clsXMLnode::strJSON(objJSON)
                                      ,strScriptWithCall = static_cast<QString>(strScript)
                                                                        + static_cast<QString>(strCall)
                                                                        + static_cast<QString>("(" + strJSON + ");");
                               pobjScriptEng->evaluate(strScriptWithCall);
                           };
cnSignal = QObject::connect(this , &clsQtTextEdit::jsonTextChanged , func);

SPlatten

@Bonnie said in Qt Lambda reference:

auto func = [pobjScriptEng, strCall, strFile, strScript, strSID](QJsonObject objJSON) {
objJSON.insert(clsXMLnode::mscszAttrSubscriberID, strSID);
QString strJSON = clsXMLnode::strJSON(objJSON)
,strScriptWithCall = static_cast<QString>(strScript)
+ static_cast<QString>(strCall)
+ static_cast<QString>("(" + strJSON + ");");
pobjScriptEng->evaluate(strScriptWithCall);
};

Perfect, thank you!

JonB

@Bonnie
If I can chip in, now that OP seems happy.

So, you managed to do it even with parameters without having to use std::function(). I thought I read that was needed when I looked at SO posts, but perhaps that was older version of C++.

Armed with your answer, I just went back over code and did my lambda-to-be-used-many-times-in-one-function with your auto func1 = [](...) { return ...; }; and it wasn't so hard after all! So thanks very much for example.