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1.9.1
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libpqxx
The C++ client library for PostgreSQL
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Communication with the database mostly happens in a text format. When you include an integer value in a query, either you use to_string to convert it to that text format, or you pass it as a paraemeter and libpqxx does it for you under the bonnet. When you get a query result field "as a float," libpqxx converts from the text format to a floating-point type. These conversions are everywhere in libpqxx.
The conversion system supports many built-in types, but it is also extensible. You can "teach" libpqxx (in the scope of your own application) to convert additional types of values to and from PostgreSQL's string format.
This is massively useful, but it's not for the faint of heart. You'll need to specialise several templates. And, the API for doing this can change with any major libpqxx release.
If that happens, your code may fail to compile with the newer libpqxx version, and you'll have to go through the NEWS file to find the API changes. Usually it'll be a small change, like an additional function you need to implement, or a constant you need to define.
In your application, a conversion is driven entirely by a C++ type you specify. The value's SQL type on the database side has nothing to do with it. Nor is there anything in the string that would identify its type. Your code says "convert to this type" and libpqxx does it.
So, if you've SELECTed a 64-bit integer from the database, and you try to convert it to a C++ short, one of two things will happen: either the number is small enough to fit in your short and it just works, or else it throws a conversion exception. Similarly, if you try to read a 32-bit SQL int as a C++ 32-bit unsigned int, that'll work fine, unless the value happens to be negative. In such cases the conversion will throw a conversion_error.
Or, your database table might have a text column, but a given field may contain a string that looks just like a number. You can convert that value to an integer type just fine. Or to a floating-point type. All that matters to the conversion is the actual value, and the type that your code specifies.
Conversions to strings can tell the type from the arguments you pass them:
For conversions in the other direction you need to instantiate the function template explicitly:
Let's say you have some other SQL type which you want to be able to store in, or retrieve from, the database. What would it take to support that?
Sometimes you do not need complete support. You might need a conversion to a string but not from a string, or vice versa. You write out the conversion at compile time, so don't be too afraid to be incomplete. If you leave out one of these steps, it's not going to crash at run time or mess up your data. The worst that can happen is that your code won't build.
So what do you need for a complete conversion?
First off, of course, you need a C++ type. It may be your own, but it doesn't have to be. It could be a type from a third-party library, or even one from the standard library that libpqxx does not yet support.
First thing to do is specialise the pqxx::name_type() function to give the type a human-readable name. That allows libpqxx error messages and such to talk about the type in a clear way. If you don't define it, libpqxx will try to figure one out with some help from the compiler, but it may not always be easy to read. In C++26 we expect to get a standard way to query the name, but until then it's a bit messy.
Then, does your type have a built-in null value? For example, a char * can be null on the C++ side. Or some types are always null, such as nullptr. You specialise the pqxx::nullness template to specify the details.
Finally, you specialise the pqxx::string_traits template. This is where you define the actual conversions.
Let's go through these steps one by one.
You'll need a type for which the conversions are not yet defined, because the C++ type is what determines the right conversion. One type gets one pair of conversions (to/from SQL strings).
The type doesn't have to be one that you create. The conversion logic was designed such that you can build it around any type. So you can just as easily build a conversion for a type that's defined somewhere else. There's no need to include any special methods or other members inside the type itself. This is also why libpqxx can convert built-in types like int.
The library also provides conversions for std::optional<T>, std::shared_ptr<T>, and std::unique_ptr<T> (for any given T). If you have conversions for T, you'll also automatically have conversions for those.
(This is a feature that should disappear once we have introspection in the C++ language. The current expectation is that C++26 will have a standard, reliable way for the code to obtain a type's name.)
When errors happen during conversion, libpqxx will compose an error message that the application can show to the user, or write to a log. Sometimes this message will need to mention the name of the type that's being converted.
By default, this will probably work fine on some compilers, or the name may come out a little strange on other compilers, but some make the names harder to recognise. So it can help to define a name yourself.
To tell libpqxx the human-readable name of a type T, there's a function template called pqxx::name_type(). This function should exist for any given type T:
(Yes, this means that you need to define something inside the pqxx namespace.)
Define this early on in your translation unit, before any code that might cause libpqxx to need the name. That way, the libpqxx code which needs to know the type's name can see your definition.
In cases where the name is not a simple compile-time constant but needs code to compute, you may need to make its type std::string. A string_view does not maintain storage space for the text it contains. However, some code analysis tools may report false posiives when initialising such strings at initialisation time.
A struct template pqxx::nullness defines whether your type has a natural "null value" built in. For example, a std::optional instantiation has a value that neatly maps to an SQL null: the un-initialised state.
If your type has a value like that, its pqxx::nullness specialisation also provides member functions for producing and recognising null values.
The simplest scenario is also the most common: most types don't have a null value built in. There is no "null `int`" in C++. In that kind of case, just derive your nullness traits from pqxx::no_null as a shorthand: This tells libpqxx that your type has no null value of its own.
(Here again you're defining this in the pqxx namespace.)
Nullness is not part of the string_traits because as far as the string conversion system is concerned, a null is not a value! You can't convert it to or from a string. Consider what would happen if you tried to convert a null value to an SQL string: you'd get a string containing the value NULL, not the null value. Handling of nulls happens at a higher level, when passing parameters to SQL statements, or quoting-and-escaping values for inclusion in SQL statements as literal values.
If your type has a natural null value, the definition for nullness gets a little more complex than the one above:
(In writing an application, if you have a type like int that doesn't have a null value but you need to pass or parse an SQL value that may be null, you can use std::optional<int> instead. An optional without a value is a null.)
You may be wondering why there's a function to produce a null value, but also a function to check whether a value is null. Why not just compare the value to the result of null()? Because two null values may not be equal. It could be like in SQL, where NULL is not equal to anything including NULL. Or there may be several different null values. Or T may override the comparison operator to behave in some unusual way.
As a third case, your type may be one that always represents a null value. This is the case for std::nullptr_t and std::nullopt_t. In a case like that, you set nullness<TYPE>::always_null to true (as well as has_null of course), and you won't need to define any actual conversions.
This part is the most work. You can skip it for types that are always null, but of course those are extremely rare.
The APIs for doing this are designed to avoid, where opssible, the need to allocate memory on the free store (a.k.a. "the heap"). In other words, the API minimises use of new/delete, even ones hidden inside std::string, std::vector, etc. The conversion API allows you to use std::string for convenience, or fixed memory buffers for maximum speed.
Start by specialising the pqxx::string_traits template. It has conversion functions going in both directions: from a C++ value to an SQL string, and from an SQL string to a C++ value. You don't absolutely have to implement both. Generally, if the code you need compiles, you're OK for the time being. Just bear in mind that future libpqxx versions may change the API — or how it uses the API internally. In fact libpqxx 8.0 made a lot of changes compared to 7.x.
As of 8.0, this is what a full specialisation of string_traits looks like:
In your own code, you typically wouldn't call these functions yourself by the way. Instead you'd call global wrappers: pqxx::to_string(), pqxx::to_buf(), pqxx::into_buf() (it's subtly different), and pqxx::size_buffer().
Notice those ctx parameters, with a default value of {}? These hold some additional "context" information for the conversion. It has no clearly defined role, except to hold extra information that your conversion may or may not need. Passing it in this way makes it easier for libpqxx to extend the conversion API further in the future without breaking compatibility.
The name ctx is shorthand for pqxx::conversion_context const t&. There are reasonable defaults for every item in there, so it's acceptable for a caller to leave it out.
As of libpqxx 8.0, conversion_context contains:
pqxx::encoding_group enc. This tells the conversion just enough about the text's encoding do to its job. It doesn't say exactly whether the text is in UTF-8, or Latin-15, or EUC-KR, and so on. But it's enough that the code can figure out where each character begins and ends, which is all the understanding that it really needs. which is all that most conversions need.std::source_location loc. We abbreviate that type as pqxx::sl because it shows up in so many places. In case the conversion throws an exception, the exception can include this source location in its message as a debugging hint.There may be more fields in the future.
The encoding group defaults to a special group, unknown. This is good enough for dealing with plain ASCII text, which is all that most conversions need. If your conversion does need to know the text's encoding, it is up to the code calling the conversion to pass that information along.
The source location defaults to the location of the immediate calling code. But if that caller is also within libpqxx, this isn't usually very helpful to application authors trying to debug errors. Therefore we try to pass the location where control last transferred from the application into libpqxx.
Now we start implementing the functions. We start off simple: from_string parses a string as an SQL value and translates it into a C++ value of type T (your type).
The string is a std::string_view, which means it doesn't necessarily end in a terminating zero. In many situations there will still be a zero just after the string, so be sure to have your tests cover cases where there's a non-zero byte behind the string!
It's always possible that the string doesn't actually represent a T value. Mistakes happen. There can be corner cases. Maybe a value is outside the range you can reasonably support, such as when you're trying to read an SQL integer into an unsigned int and it happens to be negative. When you run into that kind of thing, throw a pqxx::conversion_error.
(Of course it's also possible that you run into some other error, so it's fine to throw different exceptions as well. But when it's definitely "this is not
the right format for a `T`," throw conversion_error.)
In this function, you convert a value of T into a string that the postgres server will understand.
The caller will provide you with a buffer where you can write the string, if you need it. But for this function it doesn't really matter where you store the string: somewhere inside the buffer, or as a string constant, or even by referring to the input value. The buffer is just there in case that you need it. Usually you do.
What you can not do is allocate some memory to store your SQL text. If buf is not large enough to store your output, throw a pqxx::conversion_overrun. This doesn't have to be exact: you can be a little pessimistic and demand a bit more space than you actually need. We'll get to the negotiation for how much buffer space you need later. Just be sure to throw the exception if there's any risk of overrunning the buffer.
Beware of locales when converting. If you use standard library features like sprintf, they may obey whatever locale is currently set on the system where the code runs. That means that a simple integer like 1000000 may come out as "1000000" on your system, but as "1,000,000" on mine, or as "1.000.000" for somebody else, and on an Indian system it may be "1,00,000". Don't let that happen, or it will confuse things. Use only non-locale-sensitive library functions. Values coming from or going to the database should be in fixed, non-localised formats.
If your type contains fields of types that libpqxx already supports, you can use the existing conversion functions for those: pqxx::from_string, pqxx::to_string, pqxx::to_buf, etc. They in turn will call the string_traits specialisations for those types. For example, if you wanted to convert a date, you might use the int conversions for each of the year, month, and date numbers.
Here you estimate how much buffer space you may need for converting a T to an SQL string. Be precise if you can, but pessimistic if you must. It's usually better to waste a few bytes of space than to spend a lot of time computing the exact buffer space you need. And failing the conversion because you under-budgeted the buffer is worst of all.
Make size_buffer a constexpr function if you can. It may sometiems allow the caller to allocate the buffer on the stack, with a size known at compile time.
When converting arrays or composite values to strings, libpqxx may need to quote values and escape any special characters. This takes time.
Some types though, such as integral or floating-point types, can never have any special characters such as quotes, commas, or backslashes in their string representations. In such cases, there's no need to quote or escape such values in SQL arrays or composite types.
If your type is like that, you can tell libpqxx about this by defining:
The code that converts this type of field to strings in an array or a composite type can then use a simpler, more efficient variant of the code. It's always safe to leave this out; it's just an optimisation for when you're completely sure that it's safe.
Make sure this definition is visible wherever you call libpqxx code that may call your conversoin.
Do not do this if a string representation of your type may contain a comma; semicolon; parenthesis; brace; quote; backslash; newline; or any other character that might need escaping.
This one you don't generally need to worry about. Read on if you're writing a type which represents raw binary data, or if you're writing a template where some specialisations may contain raw binary data.
When you call parameterised statements, or prepared statements with parameters, libpqxx needs to pass your parameters on to libpq, the underlying C-level PostgreSQL client library.
There are two formats for doing that: text and binary. In the first, we represent all values as strings in the PostgreSQL text format, and the server then converts them into its own internal binary representation. That's what those string conversions above are all about, and it's what we do for almost all types of parameters.
But we do it differently when the parameter is a contiguous series of raw bytes and the corresponding SQL type is BYTEA. There is a text format for those, but we bypass it for efficiency. In this situation the server can use the binary data in the exact same form in which you provide it, without any conversion or extra processing. The binary data for BYTEA is also twice as compact during transport.
(People sometimes ask why we can't just treat all types as binary. However the general case isn't so clear-cut. The binary formats are not documented, there are no guarantees that they will be platform-independent or that they will remain stable across postgres releases, and there's no really solid way to detect when we might get the format wrong. On top of all that, the conversions aren't necessarily as straightforward and efficient as they sound. So, for the general case, libpqxx sticks with the text formats. Raw binary data alone stands out as a clear win.)
Long story short, the machinery for passing parameters needs to know: is this parameter a binary string, or not? In the normal case it can assume "no," and that's what it does. The text format is always a safe choice; we just try to use the binary format where it's faster, and safe.
The param_format function template is what drives the decision. We specialise it for types which may be binary strings, and use the default for all other types.
"Types which _may_ be binary"? You might think we know whether a type is a binary type or not. But there are some complications with generic types.
Templates like std::shared_ptr, std::optional, and so on act like "wrappers" for another type. A std::optional<T> is binary if T is binary. Otherwise, it's not. And std::variant may contain wildly different types, so the decision can even be based on the individual object rather than just the type. If you're building support for a template of this nature, you'll probably want to implement param_format for it.
Containers are another hard case. Should we pass std::vector<T> in binary? Even when T is a binary type, we don't currently have any way to pass an array in binary format, so we always pass it as text.
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