Containing means The present invention relates to containing means, in particular retractable containing means that is used to contain objects inside cupboards. Baskets are known in the prior art that comprise a containing structure suitable for receiving objects, and connected to an extendable frame. The extendable frame is provided with side rods that are slidable on support means that is fixed to side walls or backs of a cupboard, and is provided with extendable frontal rods. These extendable frontal rods are configured in such a way as to be able to vary the width of the baskets according to the width of the spaces in which such baskets are received. The extendable frontal rods comprise a circular-section hollow body and in each of its ends a circular-section portion of an end body is received internally in a coaxial manner. The end bodies with a tubular structure are provided with curved right- angled appendages that are suitable for connecting the extendable frontal rods to the side rods. The end bodies are received inside the hollow body to an extent varying from a minimum length value to a maximum length value. The correct position of the end bodies in relation to the hollow body is maintained by fixing rings interposed between the latter. In known baskets, the containing structure is provided with tube means bent into a "U" shape that is hooked onto the extendable frame by rest means. A drawback of known baskets is that the extendable frame has poor rigidity, especially when the end bodies are projected by a significant amount outside the hollow body, in such a way as to define a maximum width of such baskets. Mechanical play between-,' the end bodies and the hollow body is inevitably caused by the geometrical shape of the sections of the latter and enables the frame to flex, thus making the baskets inside the spaces in which they are received not very stable. Another drawback of known baskets is that the fixing rings, interposed between the hollow body and the end bodies, when they are subject to stress due to the use of the baskets, do not remain in a correct position, thus giving the end bodies the possibility of moving in relation to the hollow body. A further drawback of the known baskets is that, owing to the geometry of the frame sections, the end bodies can rotate in relation to the hollow bodies around a longitudinal axis of the latter, significantly reducing the rigidity of the baskets. A still further drawback of the known baskets is that processing difficulties are associated with the tube means bent into a "U" shape that involve bending angles that are by necessity considerable in order allow for elastic recovery phenomena. A still further drawback of the known baskets is that the rest elements associated with the tube means have an inadequate rest surface on the frame and furthermore have an aesthetically rough finish. An object of the invention is to improve the containing means, in particular the containing means used to contain objects inside cupboards. Another object of the invention is to provide containing means provided with extendable frame means that has a significant rigidity. The containing means according to the invention comprises structural elements that are shaped in such a way as to couple together in an effective manner, thus substantially reducing undesired mechanical play, even when the extendable frame means is arranged in a maximum extent configuration. In this way the extendable frame means makes the containing means very stable inside the cupboards in which it is received. A further object of the invention is to provide containing means provided with extendable frame means shaped in such a way that it retains its geometrical configuration after the width has been suitably determined according to the space suitable for receiving the containing means. A still further object of the invention is to provide containing means in which the end bodies and the hollow body are shaped in such a way as to become firmly coupled to reduce undesired reciprocal rotation and such as to improve the structural rigidity of the frame means. A still further object of the invention is to provide containing means that is industrially obtainable with greater ease, thus reducing processing costs. A still further object of the invention is to provide containing means in which a containing structure is effectively connected to the extendable frame means by support elements having an aesthetically fine geometrical shape. In a first aspect of the invention, containing means is provided comprising a containing structure suitable for receiving objects, extendable frame means suitable for supporting said containing structure and provided with extendable rod elements, said extendable rod elements comprising a hollow body and at least an end body provided with at least two tubular portions, characterised in that, said hollow body is shaped in such a way as to slidably receive, inside itself, part of said at least two tubular portions. In a second aspect of the invention, containing means is provided comprising rod elements suitable for supporting a containing structure suitable for receiving objects, said containing structure being provided with tube means shaped in such a way as to be able to be connected by rest elements to said rod elements, characterised in that said rest elements are shaped in such a way as to receive inside themselves ends of said tube means arranged parallel to longitudinal axes of said rod elements. Owing to the invention it is possible to obtain containing means provided " with a very rigid extendable frame means, that comprises structural elements shaped in such a way as to be coupled together in an effective manner. In this way it is possible to substantially reduce undesired mechanical play that occurs above all when the extendable frame means is arranged in a maximum extent configuration. In this way extendable frame means is obtained that makes the containing means very stable inside the cupboards in which they are received. Owing to the invention it is possible to obtain containing means provided with extendable frame means that is able to retain its geometrical configuration even during the stress that occurs during use. Owing to the invention it is possible to obtain containing means in which the end bodies and the hollow body are shaped in such a way as to become firmly coupled to reduce undesired reciprocal rotations around a longitudinal axis of the rod elements and in such a way as to further increase the structural rigidity of the extendable frame means. Owing to the invention there is provided containing means that can be obtained industrially with greater facility and with modest processing costs. Owing to the invention containing means is supplied that is provided with a containing structure that is effectively connected to the extendable frame means by rest elements that are furthermore aesthetically pleasing. The invention can be better understood and implemented with reference to the attached drawings that show some embodiments by way of non-limitative example, in which: Figure 1 is an exploded view of extendable frame means; Figure 2 is an enlarged view of a part of the extendable frame means in Figure 1; Figure 3 is an enlarged view of a part of the extendable frame means in Figure 1 in an assembly configuration; Figure 4 is a perspective and fragmentary view of containing means comprising the extendable frame means in Figure 1; Figure 5 and Figure 6 are frontal views of the extendable frame means of Figure 1 in a minimum-width and a maximum- width configuration respectively. Figure 7 is a lower fragmentary view of part of the extendable frame means in Figure 1; Figure 8 is a section view of Figure 6 taken along the plane VII-VII; Figure 9 is a section view of Figure 6 taken along the plane VIII-VIII; Figure 10 is a perspective view of a component of the extendable frame means of Figure 1; Figure 11 is another perspective view of the component in Figure 10; Figure 12 is a frontal view of the component in Figure 10; Figure 13 is an upper perspective view of a rest element of the extendable frame means in Figure 1; Figure 14 is a side view of the rest element in Figure 13; Figure 15 is a lower perspective view of the rest element in Figure 13; Figure 16 is a perspective and fragmentary view of the frame means 2 associated with an object-holding frame; Figure 17 is a perspective and fragmentary view of the frame means 2 associated with a laundry bag. With reference to Figures 1 to 17, containing means 1 is shown that is provided with extendable frame means 2 and with a containing structure 3 (shown in Figure 4) . The frame means 2 comprises frontal rod elements 6 and side rod elements 4 provided with grooves 5 engaging in a slidable manner with support means 7 that is fixed to side walls, or backs, of a cupboard space. Each frontal rod element 6 comprises a hollow body 8 that extends along a longitudinal axis X. The hollow body 8 has a cross section that is oval and elongated along a axis Y that is perpendicular to the longitudinal axis X. At the ends of this hollow body 8 there is provided fixing means 13 shaped such as to slidably receive, along the longitudinal axis X, two end bodies 9 suitable for connecting the frontal rod element 6 to the side rod elements 4. The fixing means 13 furthermore has an aesthetic line- that reflects the style of the different components of the containing means 1 and can therefore be combined with cupboards in a pleasing manner. Each end body 9 has curved right-angled appendages 10 that engage in respective side cavities 11 of the side rod element 4 to which they are fixed by setscrews 16, and two neighbouring tubular portions 12 that slidably couple with a coupling cavity 14 obtained in the fixing means 13 and with a housing cavity 15 with an oval section provided in the hollow body 8. The fixing means 13 is provided with projecting elements 19 that are inserted into the hollow body 8 and with hooking portions 17 configured in such a way as to engage in abutment seats 18 obtained in the hollow body 8. The fixing means 13 is inserted inside the hollow body 8, in such a way that the projecting elements 19 are inside the housing cavity 15 and the hooking portions 17 couple in the abutment seats 18 of the end bodies 9. At this point the fixing means 13 is not yet firmly connected to the hollow body 8 as it is free to rotate on a plane defined by the longitudinal axis X and by the axis Y. The end bodies 9, once they are connected to the hollow body 8 and to the fixing means 13, prevent the latter from moving and can be inserted into the housing cavity 15 by an amount that depends on the width of the space in which the containing means 1 is received. The end bodies 9, after they have been correctly positioned in relation to the hollow body 8, can be firmly fixed by acting on stop screws 20 associated with the fixing means 13. In this way, the hollow bodies 8 and the end bodies 9 effectively couple in a reciprocal manner, thus reducing in a substantial manner possible and undesired relative movements. Furthermore, the frontal rod elements 6, greatly withstand the weight of objects contained in the containing means 1 and owing to the fact that they have elongated sections in the direction of the axis Y they are able to reduce the flexure that is generated on the plane defined by the axis Y and by the longitudinal axis X. The frame means 2 disclosed above enables the containing means 1 to be adapted to the inside of cupboard spaces that may have widths varying from a minimum width value A to a maximum width value B. In this way, the frame means 2 according to the invention is more adaptable to spaces of differing widths. Once the frame means 2 has been connected to the inside of a cupboard space it is possible to connect the containing structure 3 to the frontal rod elements 6. The containing structure 3 provides tube means 21 that is connected to the frame means 2 by rest elements 22. The tube means 21 is bent at a right angle so that when the frame means 2 is connected, it is arranged parallel to the longitudinal axis X, or parallel to the frontal rod elements 6. Such tube means 21 is connected to the frame means 2 by rest elements 22 shaped in such a way as to pleasingly match the style of the containing means 1 and/or of the cupboards in which they are housed. Each rest element 22 comprises a body 23 in which an abutment cavity 24 is obtained that is suitable for receiving the tube means 21. With the body 23 a hooking wall 25 is firmly connected that is configured in such a way as to partially envelop the frontal rod elements 6, and is provided within itself with a rest surface 26, which in contact with the frontal rod elements 6 supports the containing structure 3. The tube means 21, shaped in the manner that has just been disclosed, is obtainable industrially in an easy and economical manner, it furthermore enables the containing structure 3 to be separated from the frame means 2 without the latter necessarily being raised in such a way as to free itself from the frontal rod elements 6. It is in fact possible to slide the rest elements 22 along the frontal rod elements 6, in such a way that they separate from the tube means 21 thus enabling the containing structure 3 to separate from the frame means 2. With the frame means 2, for example in aluminium, it is possible to associate an object-holding frame 30 (shown in Figures 16 and 17) , which can be in wood. The object-holding frame 30 comprises a pair of rods 31 that are parallel to one another and are connected together by means of transverse rods 32. The rods 31 are provided with grooves 36 open at the bottom, by means of which they can be hooked to the frontal rod elements 6 of the frame means 2. In one case, just two transverse rods 32 can be provided arranged at opposite ends of the pair of rods 31 in such a way as to define a rectangular or square-shaped perimeter of the object-holding frame 30. In this case, the object-holding frame 30 can support a laundry bag 33 (Figure 17) that can be removed by means of handles 35. In another case, it is possible to provide a plurality of transverse rods 32 that can be spaced at uniform distances from one another and on which trousers or other objects can be hung. It is furthermore possible to provide, in addition to the transverse rods 32 that act as trouser-holders, object- holding spaces 34 that can contain objects of small dimensions. The rods 31 are provided with rest surfaces 37 shaped in such a way as to receive elongated edges 36 of the object-holding spaces 34 in such a way that the latter are supported on the object-holding frame 30. In this way, the frame means 2 is very versatile and is suitable for containing a great variety of objects.