From: Warren Young
Date: June 22 2005 2:15am
Subject: Re: RFC: Row::operator[] change
List-Archive: http://lists.mysql.com/plusplus/4554
Message-Id: <42B8C9CA.5030901@etr-usa.com>
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I just finished re-doing this patch, and it's substantially different
from my previous effort:
- Row's lookup-by-index function is now at(), not col_name(), to work
more like the Standard Library.
- I decided not to remove const_subscript_container as Row's superclass,
but instead change const_subscript_container and all its subclasses
(Row, Fields, and Result) provide at() instead of operator[](int).
- This version is a bit more comprehensive.
- This one applies to the new v2.0 alpha, so it's necessarily somewhat
different than my effort from last April.
So, last time I offered a patch on this subject for comment, the only
objections were that some people were using index-by-int for speed
reasons. I pointed out that you can still use at() (nee' col_num()) for
that.
Could those of you still interested in this try this patch against the
current svn version, and see what you think? Notice how many files this
touches: it's fairly intrusive. That's why I'm running this RFC again,
instead of checking it in.
Personally, I think it's good to have the option of indexing by name,
and find indexing by integer less compelling. If speed is your thing, I
think you should be using SSQLS instead anyway.
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Index: doc/userman/userman.xml
===================================================================
--- doc/userman/userman.xml (revision 812)
+++ doc/userman/userman.xml (working copy)
@@ -3006,6 +3006,17 @@
value_list,
without any thought of whether it made
sense.
+
+ Row objects can now be indexed by a
+ string again (e.g. row["myfield"]) as in
+ v1.7.9; lookup_by_name() was removed. The
+ underlying problem that lead to that change
+ still exists, so this is at the expense of
+ indexing by an integer. To get a field by
+ its index, use the new at() function, by
+ analogy with vector::at(). This change was
+ done because it's generally more useful to
+ index a Row by a field name.
Index: lib/fields.cpp
===================================================================
--- lib/fields.cpp (revision 805)
+++ lib/fields.cpp (working copy)
@@ -35,7 +35,7 @@
return res->num_fields();
}
-const Field & Fields::operator[] (Fields::size_type i) const
+const Field& Fields::at(Fields::size_type i) const
{
res->field_seek(i);
return res->fetch_field();
Index: lib/row.h
===================================================================
--- lib/row.h (revision 808)
+++ lib/row.h (working copy)
@@ -469,11 +469,10 @@
/// \brief Get the number of fields in the row.
size_type size() const;
- /// \brief Get the value of a field given its index.
+ /// \brief Get the value of a field given its name.
///
- /// If the array index is out of bounds, the C++ standard says that
- /// the underlying vector container should throw an exception.
- /// Whether it actually does is probably implementation-dependent.
+ /// If the field does not exist in this row, we throw a BadFieldName
+ /// exception.
///
/// Note that we return the
/// \link mysqlpp::ColData_Tmpl ColData \endlink object by value.
@@ -483,30 +482,33 @@
/// like this:
///
/// \code
- /// string s = row[2];
+ /// string s = row["myfield"];
/// \endcode
///
- /// That accesses the third field in the row, returns a temporary
+ /// That accesses myfield within the row, returns a temporary
/// ColData object, which is then automatically converted to a
/// \c std::string and copied into \c s. That works fine, but
/// beware of this similar but incorrect construct:
///
/// \code
- /// const char* pc = row[2];
+ /// const char* pc = row["myfield"];
/// \endcode
///
/// This one line of code does what you expect, but \c pc is then a
/// dangling pointer: it points to memory owned by the temporary
/// ColData object, which will have been destroyed by the time you
/// get around to actually \e using the pointer.
- const ColData operator [](size_type i) const;
+ ///
+ /// This function is rather inefficient. If that is a concern for
+ /// you, use at() or the SSQLS mechanism instead.
+ const ColData operator [](const char* field) const;
- /// \brief Get the value of a field given its field name.
+ /// \brief Get the value of a field given its index.
///
- /// This function is rather inefficient. You should use operator[]
- /// if you're using Rows directly, or SSQLS for efficient named
- /// access to row elements.
- const ColData lookup_by_name(const char*) const;
+ /// If the index is out-of-bounds, the underlying vector is supposed
+ /// to throw an exception according to the C++ Standard. Whether it
+ /// actually does this is implementation-dependent.
+ const ColData at(size_type i) const;
/// \brief Return the value of a field given its index, in raw form.
///
Index: lib/result.h
===================================================================
--- lib/result.h (revision 805)
+++ lib/result.h (working copy)
@@ -304,7 +304,7 @@
/// \brief Get the underlying Field structure given its index.
const Field& fields(unsigned int i) const
{
- return fields_[i];
+ return fields_.at(i);
}
/// \brief Returns true if the other ResUse object shares the same
@@ -412,13 +412,15 @@
{
return size_type(num_rows());
}
+
/// \brief Alias for num_rows(), only with different return type.
size_type rows() const
{
return size_type(num_rows());
}
+
/// \brief Get the row with an offset of i.
- const Row operator [](size_type i) const
+ const Row at(size_type i) const
{
data_seek(i);
return fetch_row();
Index: lib/custom.pl
===================================================================
--- lib/custom.pl (revision 805)
+++ lib/custom.pl (working copy)
@@ -262,7 +262,7 @@
$parm_simple2c_b .= ", " unless $j == $i;
$defs .= " T$j I$j;";
$defs .= "\n" unless $j == $i;
- $popul .= " s->I$j = static_cast(row[ O$j ]);";
+ $popul .= " s->I$j = static_cast(row.at(O$j));";
$popul .= "\n" unless $j == $i;
$names .= " N$j ";
$names .= ",\n" unless $j == $i;
Index: lib/field_names.cpp
===================================================================
--- lib/field_names.cpp (revision 805)
+++ lib/field_names.cpp (working copy)
@@ -39,7 +39,7 @@
reserve(num);
for (int i = 0; i < num; i++) {
- std::string p(res->fields()[i].name);
+ std::string p(res->fields().at(i).name);
str_to_lwr(p);
push_back(p);
}
Index: lib/row.cpp
===================================================================
--- lib/row.cpp (revision 805)
+++ lib/row.cpp (working copy)
@@ -35,7 +35,7 @@
return res_->num_fields();
}
-const ColData Row::operator [](size_type i) const
+const ColData Row::at(size_type i) const
{
if (!initialized_) {
if (throw_exceptions())
@@ -47,14 +47,14 @@
return ColData(data_.at(i).c_str(), res_->types(i), is_nulls_[i]);
}
-const ColData Row::lookup_by_name(const char* i) const
+const ColData Row::operator [](const char* field) const
{
- int si = res_->field_num(std::string(i));
- if (si < res_->num_fields()) {
- return (*this)[si];
+ int si = res_->field_num(std::string(field));
+ if (si < size()) {
+ return at(si);
}
else {
- throw BadFieldName(i);
+ throw BadFieldName(field);
}
}
Index: lib/fields.h
===================================================================
--- lib/fields.h (revision 805)
+++ lib/fields.h (working copy)
@@ -54,12 +54,12 @@
}
/// \brief Returns a field given its index.
- const Field& operator [](size_type i) const;
+ const Field& at(size_type i) const;
/// \brief Returns a field given its index.
- const Field& operator [](int i) const
+ const Field& at(int i) const
{
- return operator [](size_type(i));
+ return at(static_cast(i));
}
size_type size() const; ///< get the number of fields
Index: lib/resiter.h
===================================================================
--- lib/resiter.h (revision 805)
+++ lib/resiter.h (working copy)
@@ -82,7 +82,7 @@
virtual size_type size() const = 0;
/// \brief Return element at given index in container
- virtual ReturnType operator [](SizeType i) const = 0;
+ virtual ReturnType at(SizeType i) const = 0;
/// \brief Return maximum number of elements that can be stored
/// in container without resizing.
@@ -182,11 +182,11 @@
/// \brief Access the current pointed-to element within the
/// container
- ReturnType* operator ->() const { return &((*d)[i]); }
+ ReturnType* operator ->() const { return &(d->at(i)); }
/// \brief Dereference the iterator, returning the pointed-to
/// element within the container.
- ReturnType operator *() const { return (*d)[i]; }
+ ReturnType operator *() const { return d->at(i); }
/// \brief Return the an element in the container given its index
ReturnType operator [](SizeType n) const { return (*d)[n]; }
Index: Wishlist
===================================================================
--- Wishlist (revision 808)
+++ Wishlist (working copy)
@@ -60,13 +60,6 @@
o Fold RowTemplate class into Row.
- o Swap the definitions of Row::operator[] and lookup_by_name().
- That is, have the operator take a field name, and define a
- function (at()?) that takes a field index. The original problem
- solved in 1.7.10 still exists, in that modern compilers won't
- let you overload the operator on both strings and integers,
- but the string form probably is the most useful.
-
o Decide on a new soname scheme? It'd be nice if the soname had
some obvious relationship to the library version. We could
actually go with a 1:1 relationship, and simply override the
Index: examples/dbinfo.cpp
===================================================================
--- examples/dbinfo.cpp (revision 805)
+++ examples/dbinfo.cpp (working copy)
@@ -133,7 +133,7 @@
for (counter = 0; counter < columns; counter++) {
if (widths[counter]) {
cout << ' ' << setw(widths[counter]) <<
- row[counter] << ' ';
+ row.at(counter) << ' ';
}
}
cout << endl;
@@ -157,7 +157,7 @@
for (i = res.begin(); i != res.end(); ++i) {
row = *i;
for (int counter = 0; counter < columns; counter++) {
- cout << row[counter] << " ";
+ cout << row.at(counter) << " ";
}
cout << endl;
}
Index: examples/util.cpp
===================================================================
--- examples/util.cpp (revision 812)
+++ examples/util.cpp (working copy)
@@ -134,8 +134,15 @@
// specially. (See Row::operator[]'s documentation for the reason.)
// As for the rest of the fields, Row::operator[] returns a ColData
// object, which can convert itself to any standard C type.
- string item(row[0]);
- print_stock_row(item, row[1], row[2], row[3], row[4]);
+ //
+ // We index the row by field name to demonstrate the feature, and
+ // also because it makes the code more robust in the face of schema
+ // changes. Use Row::at() instead if efficiency is paramount. To
+ // maintain efficiency while keeping robustness, use the SSQLS
+ // feature, demoed in the custom* examples.
+ string item(row["item"]);
+ print_stock_row(item, row["num"], row["weight"], row["price"],
+ row["sdate"]);
}
Index: examples/custom3.cpp
===================================================================
--- examples/custom3.cpp (revision 805)
+++ examples/custom3.cpp (working copy)
@@ -71,7 +71,7 @@
// there's no point in storing the result in an STL container.
// We can store the first row directly into a stock structure
// because one of an SSQLS's constructors takes a Row object.
- stock row = res[0];
+ stock row = res.at(0);
// Create a copy so that the replace query knows what the
// original values are.
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