Edge connecting two rectangles with rounded edges without gap

Question

I am trying to draw a line between rectangles with rounded corners. However, I'd like the line to touch the corners. What happens is that the line ends as if the rectangle did not have rounded corners.

I remember I already encountered this problem, but I can not remember how I solved it (maybe I just removed the rounded corners, but this time I can not).

Does anyone have suggestion on how to remove the gap ?

N.B.: I can not draw a line to the center of the rectangle, then draw the rectangle above the line, as I will have custom endings for the line (not shown in the MWE).

N.B.2: I have to work with quite a lot of such rectangles, which are not always positioned like the MWE. I am looking for a general method, not for an ad-hoc one.

\documentclass[12pt,a4paper]{article}
\usepackage{tikz}

\usetikzlibrary{
    positioning
}

\begin{document}

\begin{tikzpicture}
    \node [draw, rectangle, rounded corners] (A) {A};
    \node [draw, rectangle, rounded corners] (B) [below left = 0.5cm and 0.25cm of A] {B};
    \draw (A) -- (B);
\end{tikzpicture}
\end{document}

Answer 1

The IMHO cleanest solution is to do what Ignasi suggests: define a new shape. Assuming you do not want to do that, you can compute the intersections between the node boundaries and a path connecting their centers. This MWE comes with a basic and a slightly more advanced way of accomplishing this. The more advanced version has a style that gives the boundary path of a node the name of the node, so you do not have to add all these name path=... by hand. It also comes with a macro (no style because we need two separate paths and \pgfextra can backfire; if you are interested in an unsafe version I can add it) that allows you to connect the nodes without gap.

\documentclass[12pt,a4paper]{article}
\usepackage{tikz}

\usetikzlibrary{intersections,positioning}

\begin{document}

\begin{tikzpicture}
    \path node [draw, rectangle, rounded corners,name path=A] (A) {A};
    \path node[draw, rectangle, rounded corners,name path=B,below left = 0.5cm and 0.25cm of A] (B)  {B};
    \path[name path=AB] (A.center) -- (B.center);
    \draw[name intersections={of=A and AB,by=iA},name intersections={of=B and
    AB,by=iB}] (iA) -- (iB);
\end{tikzpicture}
\makeatletter
\tikzset{auto name node path/.style={name path=\tikz@fig@name}}
\makeatother
\newcommand{\Connect}[3][draw]{
\path[name path=#2#3] (#2.center) -- (#3.center);
\path[#1,name intersections={of=#2 and #2#3,by=i#2},
name intersections={of=#3 and #2#3,by=i#3}] (i#2) -- (i#3);
}

\begin{tikzpicture}[nodes={auto name node path}]
    \path node [draw, rectangle, rounded corners] (A) {A};
    \path node[draw, rectangle, rounded corners,below left = 0.5cm and 0.25cm of A] (B)  {B};
    \Connect{A}{B}
\end{tikzpicture}

\end{document}

Answer 2

\documentclass[margin=3mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{positioning}

\begin{document}

\begin{tikzpicture}[
box/.style = {draw, rounded corners, outer sep=0pt},
shorten <>/.style = {shorten >=#1,shorten <=#1}
                    ]
    \node [box] (A) {A};
    \node [box, below left = 0.5cm and 0.25cm of A] (B) {B};
    \draw[shorten <>=-0.5pt] (A) -- (B);
\end{tikzpicture}
\end{document}

Line is drawn from corners of rectangle shape with not rounded corners. So you need to prolong connecting line accordingly. Seems to be for 0.5pt appropriate amount.