# Pin old Boost versions

This document will teach you how to set versions of meta-packages like boost or qt5.

What is a meta-package?
In vcpkg we call meta-packages to ports that by themselves don't install anything but that instead forward installation to another port or ports. The reasons for these meta-packages to exist are plenty: to install different versions of a library depending on platform (like the old OpenSSL port did), to allow for multiple versions to exist in the vcpkg registry at the same time (OpenCV), or to conveniently install/uninstall a catalog of related packages (Boost and Qt).

In the case of Boost, it is unlikely that a user requires all of the 140+ Boost libraries in their project. For the sake of convenience, vcpkg splits Boost into multiple sub-packages broken down to individual libraries. By doing so, users can limit the subset of Boost libraries that they depend on.

If a user wants to install all of the Boost libraries available in vcpkg, they can do so by installing the boost meta-package.

Due to the nature of meta-packages, some unexpected issues arise when trying to use them with versioning. If a user writes the following manifest file:

vcpkg.json

{
"name": "demo",
"version": "1.0.0",
"builtin-baseline": "787fe1418ea968913cc6daf11855ffd8b0b5e9d4",
"dependencies": [ "boost-tuple" ],
"overrides": [
{ "name": "boost", "version": "1.72.0" }
]
}


The resulting installation plan is:

The following packages will be built and installed:
boost-assert[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-assert\3393715b4ebe30fe1c3b68acf7f84363e611f156
boost-compatibility[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-compatibility\cda5675366367789659c59aca65fc57d03c51deb
boost-config[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-config\ca82ca1b9c1739c91f3cf42c68cee56c896ae6bd
boost-container-hash[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-container-hash\bf472c23d29c3d80b562c43471eb92cea998f372
boost-detail[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-detail\96744251f025f9b3c856a275dfc338031876777b
boost-integer[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-integer\de70ce0d1500df1eda3496c4f98f42f5db256b4a
boost-io[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-io\7bf3407372f8fc2a99321d24a0e952d44fe25bf3
boost-preprocessor[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-preprocessor\8d78b8ba2e9f54cb00137115ddd2ffec1c63c149
boost-static-assert[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-static-assert\2a41c4703c7122de25b1c60510c43edc9371f63d
boost-throw-exception[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-throw-exception\b13bdf32a20786a0165cc20205ef63765cac0627
boost-tuple[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-tuple\22e3d000a178a88992c430d8ae8a0244c7dea674
boost-type-traits[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-type-traits\8829793f6c6c913257314caa317599f8d253a5ca
boost-utility[core]:x64-windows -> 1.75.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-utility\47572946bf6a63c731b9c4142eecb8bef3d3b270
boost-vcpkg-helpers[core]:x64-windows -> 7#2 -- D:\vcpkg\buildtrees\versioning\versions\boost-vcpkg-helpers\2a21e5ab45d1ce41c185faf85dff0670ea6def1d


It is reasonable to expect that overriding boost to version 1.72.0 results in all Boost packages being pinned to version 1.72.0. However, vcpkg does not treat the boost meta-package any differently that any other port. In other words, vcpkg has no notion that boost is related to all the other boost-* libraries, other than it depends on all of them. For this reason, all the other boost packages are installed at version 1.75.0, which is the baseline version.

Below, we describe two methods to pin down Boost versions effectively.

## Method 1: Pin specific packages

Use "overrides" to force specific versions in a package-by-package basis.

vcpkg.json

{
"name": "demo",
"version": "1.0.0",
"builtin-baseline": "787fe1418ea968913cc6daf11855ffd8b0b5e9d4",
"dependencies": [ "boost-tuple" ],
"overrides": [
{ "name": "boost-core", "version": "1.72" },
{ "name": "boost-integer", "version": "1.72" },
{ "name": "boost-io", "version": "1.72" },
{ "name": "boost-tuple", "version": "1.72" }
]
}


This method allows you to quickly set the specific versions you want, but you will need to write an override for each package. Boost libraries are also heavily interdependent, which means that you may end up writing a lot of override lines.

The second method makes it easy to pin the entire Boost collection and end up with a very simple manifest file.

## Method 2: Modify baseline

An easy way to set the version for the entirety of boost is to use the "builtin-baseline" property.

As of right now, it is only possible to go back to Boost version 1.75.0 using a baseline. Since that was the contemporary Boost version when the versioning feature was merged. But, it is possible to modify the baseline to whatever you like and use that instead.

### Step 1: Create a new branch

As described in the versioning documentation. The value that goes in "builtin-baseline" is a Git commit in the vcpkg repository's history. Then it stands to reason, that if you want to customize the baseline you should be able to create a new commit with said custom baseline.

Let's start by creating a new branch to hold our modified baseline.
In the directory containing your clone of the vcpkg Git repository run:

git checkout -b custom-boost-baseline


This will create a new branch named custom-boost-baseline and check it out immediately.

### Step 2: Modify the baseline

The next step is to modify the baseline file, open the file in your editor of choice and modify the entries for the Boost libraries.

Change the "baseline" version to your desired version.
NOTE: Remember to also set the port versions to 0 (or your desired version).

\${vcpkg-root}/versions/baseline.json

...
"boost": {
-      "baseline": "1.75.0",
+      "baseline": "1.72.0",
"port-version": 0
},
"boost-accumulators": {
-      "baseline": "1.75.0",
-      "port-version": 1
+      "baseline": "1.72.0",
+      "port-version": 0
},
"boost-algorithm": {
-      "baseline": "1.75.0",
+      "baseline": "1.72.0",
"port-version": 0
},
"boost-align": {
-      "baseline": "1.75.0",
+      "baseline": "1.72.0",
"port-version": 0
},
...
"boost-uninstall: {
"baseline": "1.75.0",
"port-version": 0
},
...


Some boost- packages are helpers used by vcpkg and are not part of Boost. For example, "boost-uninstall" is a vcpkg helper to conveniently uninstall all Boost libraries, but it didn't exist for Boost version 1.72.0, in this case it is fine to leave it at 1.75.0 to avoid baseline errors (since all versions in baseline.json must have existed).

### Step 3: Commit your changes

After saving your modified file, run these commands to commit your changes:

git add versions/baseline.json
git commit -m "Baseline Boost 1.72.0"


You can set the commit message to whatever you want, just make it useful for you.

### Step 4: Get your baseline commit SHA

Once all your changes are ready, you can get the commit SHA by running:

git rev-parse HEAD


The output of that command will be the commit SHA you need to put as the "builtin-baseline" in your project's manifest file. Copy the 40-hex digits and save them to use later in your manifest file.

### Step 5: (Optional) Go back to the main repository branch

Once your changes have been commited locally, you can refer to the commit SHA regardless of the repository branch you're working on. So, let's go back to the main vcpkg repository branch.

git checkout master


{
"name": "demo",
"version": "1.0.0",
"builtin-baseline": "9b5cf7c3d9376ddf43429671282972ec4f99aa85",
"dependencies": [ "boost-tuple" ]
}


In this example, commit SHA 9b5cf7c3d9376ddf43429671282972ec4f99aa85 is the commit ID with the modified baseline. Even when a different branch (master in this case) is checked out, Git is able to find the commit as long as the branch with the modified baseline exists (the custom-boost-baseline branch we created in step 1).

We run vcpkg --feature-flags="manifests,versions" install in the directory containing our manifest file and the output looks like this:

The following packages will be built and installed:
boost-assert[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-assert\6754398591f48435b28014ca0d60e5375a4c04d1
boost-compatibility[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-compatibility\9893ff3c554575bc712df4108a949e07b269f401
boost-container-hash[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-container-hash\cc19fb0154bbef188f309f49b2664ec7623b96b6
boost-core[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-core\0eb5e20df9e267e9eca325be946f52ceb8a60229
boost-detail[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-detail\759d7c6a3f9dbaed0b0c69fa0bb764f7606bb02d
boost-integer[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-integer\173956c61a26e83b0f8b58b0baf60f06aeee637c
boost-preprocessor[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-preprocessor\86eb3938b7875f124feb845331dbe84cbab5d1c6
boost-static-assert[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-static-assert\e82d8f7f3ee07e927dc374f5a08ed6d6f4ef81f4
boost-throw-exception[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-throw-exception\64df295f7df41de4fcb219834889b126b5020def
boost-tuple[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-tuple\b3e1b01ffce6e367e4fed0a5538a8546abacb6b2
boost-type-traits[core]:x64-windows -> 1.72.0 -- D:\vcpkg\buildtrees\versioning\versions\boost-type-traits\5e44ec657660eccf4d3b2710b092dd238e1e7a2d

Notice how simple our manifest file has become, instead of having a multitude of "overrides" you can pin down all Boost packages just by setting the "builtin-baseline" to be your modified baseline commit SHA.