Storage allocation of local variables inside a block in c++

Question

I want to know at which point the compiler allocates storage for local variables inside a block. How does goto and switch jump past a constructor? :

class Tree {/*...*/}
...
void foo (int i){
if (i < 10) goto label; //illegal: cannot jump past a ctor
 Tree t (45);
 label: 
   switch (i){
      case 1:
            Tree t2 (45);
            break;
      case 2: //illegal: cannot jump past ctor 
            Tree t3 (45);
            break;
   }
}

While the above code does not work for user-defined objects it works if i replace them with built-in objects. Why is that?

Edit: Built in objects like int, char, etc. The errors i get (g++ 4.5 on ubuntu):

jumpPastConstructor.c++: In function ‘void foo(int)’:
jumpPastConstructor.c++:26:3: error: jump to label ‘label’
jumpPastConstructor.c++:24:20: error:   from here
jumpPastConstructor.c++:25:10: error:   crosses initialization of ‘Tree t’
jumpPastConstructor.c++:31:16: error: jump to case label
jumpPastConstructor.c++:29:25: error:   crosses initialization of ‘Tree t2’

What kind of error are you talking about? This program is fine, because you never access any of the uninitialized objects. — Björn Pollex, Jun 30 '11 at 16:28
[I don't get errors](http://ideone.com/2LIme) (besides the syntax error for having a colon after label, which I fixed), can you provide a minimal working example and say what error you're getting? — trutheality, Jun 30 '11 at 16:29
@trutheality: you'll get errors if the class has a non-trivial constructor. — Mike Seymour, Jun 30 '11 at 16:32
@Mike Seymour: ah, thanks, [that](http://ideone.com/MPCe4) makes more sense. — trutheality, Jun 30 '11 at 16:38
@trutheality: That may be one of compiler's optimization for empty classes. Now its showing error. http://ideone.com/chBKz — Terminal, Jun 30 '11 at 16:39
see also http://stackoverflow.com/questions/2392655/what-are-the-sins-of-crosses-initialization — Cubbi, Jun 30 '11 at 16:41

score 13 · Accepted Answer · answered Jun 30 '11 at 16:37

6.7/3:

It is possible to transfer into a block, but not in a way that bypasses declarations with initialization. A program that jumps from a point where a local variable with automatic storage duration is not in scope to a point where it is in scope is ill-formed unless the variable has POD type (3.9) and is declared without an initializer (8.5).

What matters is not when the storage is allocated, but when the constructor is called. A goto that jumped past a constructor would be a problem, which is why it's banned. (POD types with no initialiser don't need any construction, so they're allowed.)

@Tuxiaomi Probably C++11; from the date it can't have been a later one. — Alan Stokes, Oct 13 '16 at 06:53

score 4 · Answer 2 · answered Jun 30 '11 at 16:34

4

the first part of the question is easy: most compilers collate all local allocations into a single stack allocation and then partition off that allocation. the initialization happens only when they come into scope or they are explicitly initialized.

Your example is pretty bad from a coding point of view, as you jump over the point at which x comes into scope, thus the constructor will never be called (this is one of the reasons why goto is bad) and why your compiler is telling you to stop trying to abuse it. However, certain types can be left uninitialized, such as the built-in types of int, float etc. you'll instead get a warning, which is why not everything throws an error if you jump over its initialization (constructor).

answered Jun 30 '11 at 16:34

Necrolis

25,836
3
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This is not one of the reasons `goto` is bad. You cannot do what you are describing. It is impossible. It is not a flaw of `goto` that you can jump over the construction of an object of class type, when `goto` does **not** let you do this (as you pointed out yourself). Perhaps you're thinking of `longjmp`. – Lightness Races in Orbit Jun 30 '11 at 16:38
@Tomalak: it lets you jump over initialization of certain types, like pointers, that **is bad**. (just tested this in MSVC). – Necrolis Jun 30 '11 at 16:43
If MSVC allows that, then MSVC is non-compliant in this regard. Alan posted the relevant paragraph from the standard, which is very clear: it is [not allowed](http://codepad.org/nuTt5x7r) (though this example has truncated the error output! >.<). But, you can go ahead and repeat your "goto is bad" mantra as long as you like without a baseful rationale: everybody else seems to. – Lightness Races in Orbit Jun 30 '11 at 16:45
@Tomalak: well then its bad to use `goto` like this under MSVC :P, stuff like this should still be avoided however... also, I never said `goto` was fully bad, I just said this is one of the cases where its bad, it has good points to it too (plus its also not my 'mantra'). as for the example it just tested my own in gcc :) – Necrolis Jun 30 '11 at 16:51

score 3 · Answer 3 · answered Jun 30 '11 at 16:45

Converted into compilable code in xx.cpp:

class C
{
    int i;
public:
    C(int i_val = 0) : i(i_val) { }
};

int main()
{
    int someval = 2;
    goto label; //error
    C x;
label:
    switch (someval)
    {
        case 1:
            C x2;
            break;
        case 2: //error
            C x3;
            break;
    }
}

and compiled as shown with G++ 4.6.0 on MacOS X 10.6.8 yields the errors shown:

$ g++ -Wall -Wextra -c xx.cpp
xx.cpp: In function ‘int main()’:
xx.cpp:13:1: error: jump to label ‘label’ [-fpermissive]
xx.cpp:11:10: error:   from here [-fpermissive]
xx.cpp:12:7: error:   crosses initialization of ‘C x’
xx.cpp:19:14: error: jump to case label [-fpermissive]
xx.cpp:17:15: error:   crosses initialization of ‘C x2’
$

There is a default constructor for each of the variables x, x2, and x3.

And the C++ standard simply says you are not allowed to jump into a block past variable construction. What would work is:

class C
{
    int i;
public:
    C(int i_val = 0) : i(i_val) { }
};

int main()
{
    int someval = 2;
    goto label; //error
    {
    C x;
    }
label:
    switch (someval)
    {
        case 1:
            {
            C x2;
            }
            break;
        case 2: //error
            {
            C x3;
            }
            break;
    }
}

With the three extra pairs of braces, you are no longer jumping into the blocks where the variables are declared and initialized, so the code is legitimate and compiles cleanly under the command line shown before.

score 2 · Answer 4 · answered Feb 18 '14 at 04:22

2

Solution here is: Add bracket to each CASE

switch (i){
  case 1:{
        Tree t2 (45);
        break;
  }
  case 2: {//illegal: cannot jump past ctor 
        Tree t3 (45);
        break;
  }

}

I don't know what crazy is this!!!! But Add { and } could solve this issue!

answered Feb 18 '14 at 04:22

Phuong

1,153
1
10
17

"Because transfer of control is not permitted to enter the scope of a variable, if a declaration statement is encountered inside the statement, it has to be scoped in its own compound statement:" explains the [docs](https://en.cppreference.com/w/cpp/language/switch). I hope it is relevant :D – numan May 24 '22 at 07:20

score 0 · Answer 5 · answered Jul 13 '16 at 08:23

0

I came here because I had the same problem.
What helped for me was to decare the 'Tree' outside the case.
And to not Tree t2 (45); but to place Tree t2; before switch.
And t2 (45); inside case 1.

And then you need to do the same for Tree t3 (45);.

answered Jul 13 '16 at 08:23

Tenzin

2,415
2
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score 0 · Answer 6 · answered Jun 30 '11 at 16:32

You can't goto or case across constructors. Builtins don't have constructors.

The compiler knows when it enters the function what the total space requirement of its local variables is, and it will move the stack pointer to accommodate them. This memory is uninitialised.

It calls the constructors and destructors during the function flow as it needs to. This is why you can't use goto or case like that -- it breaks the invariants. Statements such as break call destructors as necessary in, say, a for-loop, and everything works out OK.

Storage allocation of local variables inside a block in c++

6 Answers6

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