Using commands inside environment definitions

Question

I want to do something like

\newenvironment{definition}
{
    \textit{
}
{
    }
}

but of course it doesn't work, the braces get mixed up. (In other words, I want that within my environment, the content is surrounded by a command such as \textit or \textbf for instance.) Any ideas?

EDIT : My environment now looks like this :

\newenvironment{definition}
{
    \textbf{\underline{Definition.}}
    \vspace{12 pt}
    \begin{itshape}
}
{
    \end{itshape}
}

Answer 1

In your given example, you can define

\newenvironment{myenv}{\itshape}{}

Actually, the way environments work, you could just use

\begin{itshape} <content> \end{itshape}

but don’t, it is better to have environments named in a meaningful semantic way, say

\newenvironment{emphasize}{\itshape}{}
\begin{emphasize} <content> \end{emphasize}

---

If you really need to use a macro that expects a parameter (like \textit) you can use the environ package and its \BODY macro as demonstrated by Werner.

Answer 2

In a general setting, you wish to capture the entire environment into a macro, and then pass that macro to another macro as an argument. That is exactly what the environ package provides:

\documentclass{article}
\usepackage{environ}% http://ctan.org/pkg/environ
\NewEnviron{myenv}[1][\textit]{%
  #1{\BODY}%
}
\begin{document}
\begin{myenv}
Hello World!
\end{myenv}

\begin{myenv}[\textbf]
Hello World!
\end{myenv}

\begin{myenv}[\slshape]
Hello World!
\end{myenv}

\begin{myenv}[]
Hello World!
\end{myenv}
\end{document}

environ allows you to capture the contents of the environment into the macro \BODY, which is then usable for whatever means. My definition prints it using \textit by default, but you can modify that using a different macro or font switch.

The above definition of myenv is general enough to allow for formatting of the environment content using a macro or a switch (as is done in the 3rd example). Supplying an empty (optional) argument removes any formatting.

Answer 3

\documentclass{article}
\newenvironment{myenv}{\fontshape{it}\selectfont}{\fontshape{n}\selectfont}
\begin{document}
Before the environment

\begin{myenv}
Testing in the environment
\end{myenv}

Out of the environment
\end{document}