Strange multiple assignment error in C++

问题

I have multiple assignment statement in my program as shown below where query.constraints.size() is supposed to return 13 (constraints is an array and its returning its size)

int num,size =  query.constraints.size();

When I do this size becomes 13 as expected but num becomes 9790272 for some reason.

When I do them separately as below everything is ok and both of them are 13 as expected

int size =  query.constraints.size();

int num =  query.constraints.size();

Why does my multiple assignment result in a strange a strange value ?

回答1:

Why does my multiple assignment result in a strange a strange value ?

Because C++ has no multiple assignment¹. You are declaring two variables here, but only initialise the second, not the first.

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¹ Well, you can do int a, b = a = c; but code which does this would be deemed bad by most C++ programmers except in very peculiar circumstances.

回答2:

You're not assigning multiple times, you're declaring multiple times. You need to do something like:

int num, size;
size = num = query.constraints.size();

回答3:

What you have is actually a declaration statement, partially with initializer. Your code is equivalent to this code:

int num;  // uninitialized, you're not allowed to read it

int size(query.constraints.size());  // initialized

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In general, T x = expr; declares a variable x of type T and copy-initializes it with the value of expr. For fundamental types this just does what you expect. For class-types, the copy-constructor is only formally required, but in practice usually elided.

回答4:

A mutiple assignement would looks like:

int num, size;
num = size = query.constraints.size();

But the comma operator does not do a multiple assignement.

回答5:

The comma operator doesnt do what you think it does

来源：https://stackoverflow.com/questions/11904307/strange-multiple-assignment-error-in-c