概要
為替レート取得は各FX証券会社がAPIを出していますが、口座開く必要があったり、証拠金を差し入れる必要があったりとなかなかフリーで取ってくるAPIが無いのが現状です。
ですが、外為オンラインさんがJSONで出力していて口座とか開かなくても使えそうなのでこれをつかってみましょうというお話です。
やっぱりこういったWebサービスはJSONで出力してくれたほうがパーサが使えて楽できるので、とてもありがたいですね。
JSON最高！超便利！
出力フォーマット
ここ
http://www.gaitameonline.com/rateaj/getrate
にアクセスすると、JSONでこんな感じで出てきます。
{"quotes":[{"high":"2.0130","open":"2.0125","bid":"2.0071","currencyPairCode":"GBPNZD","ask":"2.0088","low":"2.0062"},{"high":"103.90","open":"103.84","bid":"103.57","currencyPairCode":"CADJPY","ask":"103.62","low":"103.47"},{"high":"1.9165","open":"1.9146","bid":"1.9115","currencyPairCode":"GBPAUD","ask":"1.9124","low":"1.9094"},{"high":"97.95","open":"97.78","bid":"97.66","currencyPairCode":"AUDJPY","ask":"97.69","low":"97.55"},{"high":"1.0518","open":"1.0506","bid":"1.0496","currencyPairCode":"AUDNZD","ask":"1.0506","low":"1.0482"},{"high":"1.4162","open":"1.4139","bid":"1.4145","currencyPairCode":"EURCAD","ask":"1.4152","low":"1.4132"},{"high":"1.2188","open":"1.2172","bid":"1.2177","currencyPairCode":"EURUSD","ask":"1.2178","low":"1.2169"},{"high":"93.19","open":"93.00","bid":"92.98","currencyPairCode":"NZDJPY","ask":"93.04","low":"92.82"},{"high":"1.1626","open":"1.1614","bid":"1.1614","currencyPairCode":"USDCAD","ask":"1.1620","low":"1.1606"},{"high":"0.7849","open":"0.7841","bid":"0.7845","currencyPairCode":"EURGBP","ask":"0.7849","low":"0.7840"},{"high":"1.5526","open":"1.5515","bid":"1.5516","currencyPairCode":"GBPUSD","ask":"1.5519","low":"1.5503"},{"high":"10.296","open":"10.283","bid":"10.252","currencyPairCode":"ZARJPY","ask":"10.402","low":"10.247"},{"high":"1.2026","open":"1.2021","bid":"1.2022","currencyPairCode":"EURCHF","ask":"1.2027","low":"1.2020"},{"high":"122.22","open":"122.14","bid":"121.86","currencyPairCode":"CHFJPY","ask":"121.91","low":"121.74"},{"high":"0.8123","open":"0.8103","bid":"0.8115","currencyPairCode":"AUDUSD","ask":"0.8117","low":"0.8083"},{"high":"0.9880","open":"0.9876","bid":"0.9871","currencyPairCode":"USDCHF","ask":"0.9875","low":"0.9865"},{"high":"146.99","open":"146.88","bid":"146.55","currencyPairCode":"EURJPY","ask":"146.57","low":"146.43"},{"high":"1.5329","open":"1.5325","bid":"1.5317","currencyPairCode":"GBPCHF","ask":"1.5326","low":"1.5305"},{"high":"1.5790","open":"1.5784","bid":"1.5752","currencyPairCode":"EURNZD","ask":"1.5764","low":"1.5746"},{"high":"0.7729","open":"0.7706","bid":"0.7725","currencyPairCode":"NZDUSD","ask":"0.7731","low":"0.7702"},{"high":"120.75","open":"120.67","bid":"120.35","currencyPairCode":"USDJPY","ask":"120.36","low":"120.27"},{"high":"1.5032","open":"1.5014","bid":"1.5000","currencyPairCode":"EURAUD","ask":"1.5008","low":"1.4985"},{"high":"0.8019","open":"0.8002","bid":"0.8010","currencyPairCode":"AUDCHF","ask":"0.8016","low":"0.7988"},{"high":"187.37","open":"187.26","bid":"186.74","currencyPairCode":"GBPJPY","ask":"186.77","low":"186.61"}]}
調べたところ、更新頻度はだいたい1秒単位のようです。
ただ、1秒おきにアクセスするのはどうかと思うので、そのへんは良心にお任せということで。。
（もっと細かいtick単位で取りたければ、証拠金入れてちゃんと口座開くなり、デモ口座開くなりして、正式なAPI使ってやりましょうという話）
あと急な変動があった場合、一時的に取得に失敗するとかありましたのでその辺りも注意。
C++でのサンプル
簡単なサンプルですが、以下に置きました。
https://github.com/chromabox/jpyrate
サンプルではUSD/JPY（ドル円）のレートを１分おきに取得するようにしています。
bid:120.35 ask:120.36
bid:120.32 ask:120.33
bid:120.33 ask:120.34
bid:120.32 ask:120.33
bid:120.32 ask:120.33
売り値：買い値 の順番で表示します。
Ctrl+Cで終わります。
注意
正規の方法では無いので、頻繁にアクセスするのは控えましょう。
個人で使う分にはまぁ良いと思いますが、データを商用利用するとか、そういうのはまずいと思います。
参考文献
Googleの為替レートAPIが利用できなくなっていた件
http://8wired.jp/blog/1121