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@hskoglund said in Trouble assigning value to boolean in the struct:
it's because the const
QVector::value() does not return a const object!
wrote on 24 Jan 2020, 22:11 last edited by@Christian-Ehrlicher Hmmm, but the docs says that API is const:
T QVector::value(int i) const -
@JonB said in Trouble assigning value to boolean in the struct:
it should produce the same error.
In theory it's a temporary so it should compile. In practice the compiler prevents you from shooting into your foot - at least when the left hand side is a POD. As soon as it's a class the compiler will not complain - blame the compiler :)
struct blub { int a = 0; int b = 0; blub(int _a, int _b) : a(_a), b(_b) {} blub() = default; }; struct foo { std::string stdStr; QString str; QPointF p; blub myBlub; int myInt = -1; bool myBool = false; }; QVector<foo> fooVec; void doSomething() { int i = 5; fooVec.value(i).stdStr = "str"; fooVec.value(i).str = "str"; fooVec.value(i).p = QPointF(1,2); fooVec.value(i).myBlub = blub(1,2); fooVec.value(i).myInt = 2; // gives compiler error fooVec.value(i).myBool = true; // dito }@Christian-Ehrlicher said in Trouble assigning value to boolean in the struct:
As soon as it's a class the compiler will not complain - blame the compiler
But don't rush to. The compiler has to generate the assignment for the trivial bool, hence it can see that the written is nonsense. On the other hand
QStringhas both assignment operators defined already, so the compiler has to be really, really, really clever to understand that the written assignment is not what was intended. (for example the compiler has to verify that the destructor of mentioned temporary produces no side effects, which isn't trivial to begin with, to be able to then to infer that the assignment is wrong) -
@Christian-Ehrlicher Hmmm, but the docs says that API is const:
T QVector::value(int i) const@hskoglund said in Trouble assigning value to boolean in the struct:
but the docs says that API is const
The point, Henry, is not that the method is non-mutating, which is without a doubt correct, but that it returns a copy of the element, which copy is then modified.
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@hskoglund said in Trouble assigning value to boolean in the struct:
but the docs says that API is const
The point, Henry, is not that the method is non-mutating, which is without a doubt correct, but that it returns a copy of the element, which copy is then modified.
wrote on 24 Jan 2020, 22:17 last edited by@kshegunov Aha, you're right! If the method had a const at the beginning things would be different :-)
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@Christian-Ehrlicher said in Trouble assigning value to boolean in the struct:
As soon as it's a class the compiler will not complain - blame the compiler
But don't rush to. The compiler has to generate the assignment for the trivial bool, hence it can see that the written is nonsense. On the other hand
QStringhas both assignment operators defined already, so the compiler has to be really, really, really clever to understand that the written assignment is not what was intended. (for example the compiler has to verify that the destructor of mentioned temporary produces no side effects, which isn't trivial to begin with, to be able to then to infer that the assignment is wrong)@kshegunov I thought this too, but even when the compiler generates all the operators (see my struct blub) it will not complain. So I think they simply did not implement it for classes.
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@kshegunov I thought this too, but even when the compiler generates all the operators (see my struct blub) it will not complain. So I think they simply did not implement it for classes.
@Christian-Ehrlicher said in Trouble assigning value to boolean in the struct:
@kshegunov I thought this too, but even when the compiler generates all the operators (see my struct blub) it will not complain. So I think they simply did not implement it for classes.
Yes probably, and probably because it's nontrivial in the general case. While at the same time for an intrinsic type it's plain and simple to deduce.
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@kshegunov Aha, you're right! If the method had a const at the beginning things would be different :-)
Lifetime Qt Championwrote on 24 Jan 2020, 22:21 last edited by Christian Ehrlicher@hskoglund said in Trouble assigning value to boolean in the struct:
had a const at the beginning
But then the copy elision would not work afair. -
wrote on 24 Jan 2020, 22:41 last edited by hskoglund
Thinking about this, if the intent is to modify the elements of the QVector, then using value() is a bad idea since you're working with a copy (even though it happens to work for QStrings in that struct).
But if the API was changed to have another const at the beginning, say:
const T QVector::value(int i) constI'm too lazy to test it, but wouldn't that leading const cause compilation errors also for QStrings assignments? If so that would help dispel confusion about the usage of value().
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Thinking about this, if the intent is to modify the elements of the QVector, then using value() is a bad idea since you're working with a copy (even though it happens to work for QStrings in that struct).
But if the API was changed to have another const at the beginning, say:
const T QVector::value(int i) constI'm too lazy to test it, but wouldn't that leading const cause compilation errors also for QStrings assignments? If so that would help dispel confusion about the usage of value().
@hskoglund said in Trouble assigning value to boolean in the struct:
I'm too lazy to test it, but wouldn't that leading const cause compilation errors also for QStrings assignments?
It would. I'm not sure if the compiler can RVO it correctly if you assign to a non-const however ... that needs actual looking at the asm output.
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From my pov QVector::value() should be deprecated - it's a really weird function which returns a copy just for the reason that the caller does not do the bounds checking.
I don't see a difference in the asm output in a single testcase. -
wrote on 27 Jan 2020, 10:10 last edited by
Thank you for the explanation - indeed, I saw in docs that value() is const but since it worked for the QString I just... assumed it works anyway. Silly of me.
So, to make it work as intended I should use [index] notation, like @hskoglund said (I am somehow opposed to using it in the favour of methods).
Since I already got everyones involved attention - the QVector (in this case) is supposed to hold less than 20 of items, set once and not being manipulated further. Is the choice of QVector over other container types a valid one? I read about container classes and differences between them and after that I rather tend to avoid QList.
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Thank you for the explanation - indeed, I saw in docs that value() is const but since it worked for the QString I just... assumed it works anyway. Silly of me.
So, to make it work as intended I should use [index] notation, like @hskoglund said (I am somehow opposed to using it in the favour of methods).
Since I already got everyones involved attention - the QVector (in this case) is supposed to hold less than 20 of items, set once and not being manipulated further. Is the choice of QVector over other container types a valid one? I read about container classes and differences between them and after that I rather tend to avoid QList.
@artwaw said in Trouble assigning value to boolean in the struct:
after that I rather tend to avoid QList.
👍 very good.
If you want to stay in Qt - World, than there are not many other options available.
A simple c/c++ array could be enough for your case? -
@artwaw said in Trouble assigning value to boolean in the struct:
after that I rather tend to avoid QList.
👍 very good.
If you want to stay in Qt - World, than there are not many other options available.
A simple c/c++ array could be enough for your case?wrote on 27 Jan 2020, 10:22 last edited by@J-Hilk Might be enough, of course. QVector seemed... just convenient.
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wrote on 27 Jan 2020, 10:24 last edited by
Thank you all, I learned a lot from my mistake. Will pay more attention to docs and implications next time.
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wrote on 27 Jan 2020, 12:36 last edited by JonB
Now that the OP appears to be happy he is solved, I'd like to say that I am not! :) And would ask for further explanation.
I wrote earlier:
@artwaw said in Trouble assigning value to boolean in the struct:
If const is the cause of the error (and I don't doubt it is) forcurrent.fields.value(ui->editSectionNum->currentIndex()).permanent=(state>0);`why is there no error for
current.fields.value(ui->editSectionNum->currentIndex()).pattern=ui->sectionEdit->text();? That was the OP's query. That's what I don't see, it should produce the same error.
@hskoglund replied
I suspect it's because the
constis only 1 layer deep, i.e. it protects integers and booleans in that struct from being reassigned, but since QStrings are pointers, that const only protects reassignment of the pointer, not what the pointer points to, i.e. QStrings data().I know the
const"is only one level deep". And I don't understand how he comes up with that "the pointer value will be retained but what it points to will be changed".And @Christian-Ehrlicher wrote
In theory it's a temporary so it should compile.
I don't understand either of these. The OP's
structhasQString pattern; bool permanent;I have yet to understand why the
boolcan be altered but not theQString. (Yes, I knowQStrings are stored shared, so what?) They are both members of thestruct. Ifcurrent.fields.value()returns aconstthen neither of them should be alterable, and if it does not both should be alterable.Would somebody care to enlighten me on what exactly is the difference?
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Now that the OP appears to be happy he is solved, I'd like to say that I am not! :) And would ask for further explanation.
I wrote earlier:
@artwaw said in Trouble assigning value to boolean in the struct:
If const is the cause of the error (and I don't doubt it is) forcurrent.fields.value(ui->editSectionNum->currentIndex()).permanent=(state>0);`why is there no error for
current.fields.value(ui->editSectionNum->currentIndex()).pattern=ui->sectionEdit->text();? That was the OP's query. That's what I don't see, it should produce the same error.
@hskoglund replied
I suspect it's because the
constis only 1 layer deep, i.e. it protects integers and booleans in that struct from being reassigned, but since QStrings are pointers, that const only protects reassignment of the pointer, not what the pointer points to, i.e. QStrings data().I know the
const"is only one level deep". And I don't understand how he comes up with that "the pointer value will be retained but what it points to will be changed".And @Christian-Ehrlicher wrote
In theory it's a temporary so it should compile.
I don't understand either of these. The OP's
structhasQString pattern; bool permanent;I have yet to understand why the
boolcan be altered but not theQString. (Yes, I knowQStrings are stored shared, so what?) They are both members of thestruct. Ifcurrent.fields.value()returns aconstthen neither of them should be alterable, and if it does not both should be alterable.Would somebody care to enlighten me on what exactly is the difference?
wrote on 27 Jan 2020, 12:47 last edited byI'd be happy to read further on the details too, if our more experienced colleagues don't mind?
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Now that the OP appears to be happy he is solved, I'd like to say that I am not! :) And would ask for further explanation.
I wrote earlier:
@artwaw said in Trouble assigning value to boolean in the struct:
If const is the cause of the error (and I don't doubt it is) forcurrent.fields.value(ui->editSectionNum->currentIndex()).permanent=(state>0);`why is there no error for
current.fields.value(ui->editSectionNum->currentIndex()).pattern=ui->sectionEdit->text();? That was the OP's query. That's what I don't see, it should produce the same error.
@hskoglund replied
I suspect it's because the
constis only 1 layer deep, i.e. it protects integers and booleans in that struct from being reassigned, but since QStrings are pointers, that const only protects reassignment of the pointer, not what the pointer points to, i.e. QStrings data().I know the
const"is only one level deep". And I don't understand how he comes up with that "the pointer value will be retained but what it points to will be changed".And @Christian-Ehrlicher wrote
In theory it's a temporary so it should compile.
I don't understand either of these. The OP's
structhasQString pattern; bool permanent;I have yet to understand why the
boolcan be altered but not theQString. (Yes, I knowQStrings are stored shared, so what?) They are both members of thestruct. Ifcurrent.fields.value()returns aconstthen neither of them should be alterable, and if it does not both should be alterable.Would somebody care to enlighten me on what exactly is the difference?
@JonB said in Trouble assigning value to boolean in the struct:
If current.fields.value() returns a const then neither of them should be alterable, and if it does not both should be alterable.
Look carefully at what I wrote to Henry. The struct's instance returned by
QVector::valueis not const. It's a temporary (probably the compiler does it as an rvalue), so it's valid to write its members. However the compiler deduces that assigning to the boolean of said temporary is meaningless, thus as @Christian-Ehrlicher eloquently put it - it prevents you from shooting yourself in the foot. When it comes to classes and structs it's more complicated, which we discussed briefly with Christian upstairs. -
@JonB said in Trouble assigning value to boolean in the struct:
If current.fields.value() returns a const then neither of them should be alterable, and if it does not both should be alterable.
Look carefully at what I wrote to Henry. The struct's instance returned by
QVector::valueis not const. It's a temporary (probably the compiler does it as an rvalue), so it's valid to write its members. However the compiler deduces that assigning to the boolean of said temporary is meaningless, thus as @Christian-Ehrlicher eloquently put it - it prevents you from shooting yourself in the foot. When it comes to classes and structs it's more complicated, which we discussed briefly with Christian upstairs.wrote on 27 Jan 2020, 16:10 last edited by JonB@kshegunov said in Trouble assigning value to boolean in the struct:
instance returned by QVector::value is not const. It's a temporary
Good, got that (was always dubious about whether it was
constanyway).However the compiler deduces that assigning to the boolean of said temporary is meaningless
Ah ha!! Got it! Sooooooooo... what you are saying is: on the one hand we now have a compiler which gets involved in the meaning of life/teleological purpose, yet on the other hand chooses to blurt out "Expression not assignable" as the diagnostic message when it's non-such. "Meaningless assignment", "Assignment with no side-effect" would have been much better, "non-assignable" means e.g. it's
constto me, hence my confusion (and perhaps that of @artwaw).What happened to the days when I wrote code and the compiler knew its place, minded its business and just shut up and compiled?
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@kshegunov said in Trouble assigning value to boolean in the struct:
instance returned by QVector::value is not const. It's a temporary
Good, got that (was always dubious about whether it was
constanyway).However the compiler deduces that assigning to the boolean of said temporary is meaningless
Ah ha!! Got it! Sooooooooo... what you are saying is: on the one hand we now have a compiler which gets involved in the meaning of life/teleological purpose, yet on the other hand chooses to blurt out "Expression not assignable" as the diagnostic message when it's non-such. "Meaningless assignment", "Assignment with no side-effect" would have been much better, "non-assignable" means e.g. it's
constto me, hence my confusion (and perhaps that of @artwaw).What happened to the days when I wrote code and the compiler knew its place, minded its business and just shut up and compiled?
@JonB said in Trouble assigning value to boolean in the struct:
What happened to the days when I wrote code and the compiler knew its place, minded its business and just shut up and compiled?
They've passed some time ago. The compilers've become smarter than us (developers) so we only can bite the bullet on that one, sorry. I expect soon compilers to be writing compilers, Terminator 2-style ...
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wrote on 28 Jan 2020, 10:39 last edited by
@kshegunov @Christian-Ehrlicher I have another question: since value() returns const but copy of QString occurs (re my very unfortunate construction with assignment to QString) - will the value assigned to that QString remain in the QVector? My understanding is that not, after terminating the method copy would be discarded and new value lost. Do I understand correctly?
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